Clear app data via the Activity Manager, not Package Manager

This is because doing it through the Activity Manager also makes sure that the
app has been shut down first, which is quite a desirable invariant for restore
handling.

services/java/com/android/server/BackupManagerService.java

@@ -792,7 +792,11 @@ class BackupManagerService extends IBackupManager.Stub {
 
         synchronized(mClearDataLock) {
             mClearingData = true;
-            mPackageManager.clearApplicationUserData(packageName, observer);
+            try {
+                mActivityManager.clearApplicationUserData(packageName, observer);
+            } catch (RemoteException e) {
+                // can't happen because the activity manager is in this process
+            }
 
             // only wait 10 seconds for the clear data to happen
             long timeoutMark = System.currentTimeMillis() + TIMEOUT_INTERVAL;