Add a listener when task snapshots change

Since we start recents before we take the snapshot, we need to add
a mechanism to inform recents about task snapshots changes.

We add a new method to TaskStackChangedListener,
onTaskSnapshotChanged, which gets called whenever a task snapshot
changes. Then, SystemUI registers such a listener and updates the
task thumbnail view for the specific task.

Test: Open app, press recents, make sure thumbnail is up-to-date
Bug: 31339431
Change-Id: I01e81b9cd11886da734da671c68d5732aa51009f
diff --git a/services/tests/servicestests/src/com/android/server/wm/WindowTestsBase.java b/services/tests/servicestests/src/com/android/server/wm/WindowTestsBase.java
index 466bd67..25f2c6e 100644
--- a/services/tests/servicestests/src/com/android/server/wm/WindowTestsBase.java
+++ b/services/tests/servicestests/src/com/android/server/wm/WindowTestsBase.java
@@ -19,6 +19,8 @@
 import org.junit.Assert;
 import org.junit.Before;
 
+import android.app.ActivityManager;
+import android.app.ActivityManager.TaskSnapshot;
 import android.content.Context;
 import android.os.IBinder;
 import android.support.test.InstrumentationRegistry;
@@ -208,7 +210,7 @@
     class TestTaskWindowContainerController extends TaskWindowContainerController {
 
         TestTaskWindowContainerController(int stackId) {
-            super(sNextTaskId++, stackId, 0 /* userId */, null /* bounds */,
+            super(sNextTaskId++, snapshot -> {}, stackId, 0 /* userId */, null /* bounds */,
                     EMPTY /* overrideConfig*/, RESIZE_MODE_UNRESIZEABLE, false /* homeTask*/,
                     false /* isOnTopLauncher */, true /* toTop*/, true /* showForAllUsers */);
         }