Don't crash in various ways when using backup services too early

BackupManager now no longer tries to use a null service binder if it's used
early during the boot process.  ActivityManagerService no longer tries to
dereference null pointers if bind/unbind semantics get out of step due to things
being run too early.
diff --git a/services/java/com/android/server/am/ActivityManagerService.java b/services/java/com/android/server/am/ActivityManagerService.java
index fd1dfc8..6d04b6b 100644
--- a/services/java/com/android/server/am/ActivityManagerService.java
+++ b/services/java/com/android/server/am/ActivityManagerService.java
@@ -10472,8 +10472,17 @@
     // done with this agent
     public void unbindBackupAgent(ApplicationInfo appInfo) {
         if (DEBUG_BACKUP) Log.v(TAG, "unbindBackupAgent: " + appInfo);
+        if (appInfo == null) {
+            Log.w(TAG, "unbind backup agent for null app");
+            return;
+        }
 
         synchronized(this) {
+            if (mBackupAppName == null) {
+                Log.w(TAG, "Unbinding backup agent with no active backup");
+                return;
+            }
+
             if (!mBackupAppName.equals(appInfo.packageName)) {
                 Log.e(TAG, "Unbind of " + appInfo + " but is not the current backup target");
                 return;