Fix Bluetooth GATT API default handler assignment

Restores previous behaviour where GATT callbacks are invoked on the
binder thread and not the calling process main looper thread.

This fixes performance regressions as well as some
NetworkOnMainThreadException's for some applications.

Bug: 37544152
Bug: 37871717
Test: Covered by prior API tests.
Change-Id: Id8ab705dd4d7f00030e6ac29e056dde5180670e9
diff --git a/core/java/android/bluetooth/BluetoothGatt.java b/core/java/android/bluetooth/BluetoothGatt.java
index 0f01d62..5fabbb6 100644
--- a/core/java/android/bluetooth/BluetoothGatt.java
+++ b/core/java/android/bluetooth/BluetoothGatt.java
@@ -156,7 +156,7 @@
                 }
                 mClientIf = clientIf;
                 if (status != GATT_SUCCESS) {
-                    mHandler.post(new Runnable() {
+                    runOrQueueCallback(new Runnable() {
                         @Override
                         public void run() {
                             if (mCallback != null) {
@@ -191,7 +191,7 @@
                     return;
                 }
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -213,7 +213,7 @@
                     return;
                 }
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -238,7 +238,7 @@
                 int profileState = connected ? BluetoothProfile.STATE_CONNECTED :
                                                BluetoothProfile.STATE_DISCONNECTED;
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -300,7 +300,7 @@
                     }
                 }
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -352,7 +352,7 @@
 
                 if (status == 0) characteristic.setValue(value);
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -401,7 +401,7 @@
 
                 mAuthRetryState = AUTH_RETRY_STATE_IDLE;
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -430,7 +430,7 @@
 
                 characteristic.setValue(value);
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -477,7 +477,7 @@
 
                 mAuthRetryState = AUTH_RETRY_STATE_IDLE;
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -523,7 +523,7 @@
 
                 mAuthRetryState = AUTH_RETRY_STATE_IDLE;
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -549,7 +549,7 @@
                     mDeviceBusy = false;
                 }
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -570,7 +570,7 @@
                 if (!address.equals(mDevice.getAddress())) {
                     return;
                 }
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -592,7 +592,7 @@
                     return;
                 }
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -616,7 +616,7 @@
                     return;
                 }
 
-                mHandler.post(new Runnable() {
+                runOrQueueCallback(new Runnable() {
                     @Override
                     public void run() {
                         if (mCallback != null) {
@@ -703,6 +703,22 @@
     }
 
     /**
+     * Queue the runnable on a {@link Handler} provided by the user, or execute the runnable
+     * immediately if no Handler was provided.
+     */
+    private void runOrQueueCallback(final Runnable cb) {
+        if (mHandler == null) {
+          try {
+            cb.run();
+          } catch (Exception ex) {
+            Log.w(TAG, "Unhandled exception in callback", ex);
+          }
+        } else {
+          mHandler.post(cb);
+        }
+    }
+
+    /**
      * Register an application callback to start using GATT.
      *
      * <p>This is an asynchronous call. The callback {@link BluetoothGattCallback#onAppRegistered}