Add callbacks for MediaProjection start / stop.

Also, enforce that there is only one valid MediaProjection at a time.

Bug: 16488053
Change-Id: Id05445d798c98cb208bc4dab186296392e15d30b
diff --git a/Android.mk b/Android.mk
index b419231..1cdc709 100644
--- a/Android.mk
+++ b/Android.mk
@@ -329,6 +329,7 @@
 	media/java/android/media/projection/IMediaProjection.aidl \
 	media/java/android/media/projection/IMediaProjectionCallback.aidl \
 	media/java/android/media/projection/IMediaProjectionManager.aidl \
+	media/java/android/media/projection/IMediaProjectionWatcherCallback.aidl \
 	media/java/android/media/routing/IMediaRouteService.aidl \
 	media/java/android/media/routing/IMediaRouteClientCallback.aidl \
 	media/java/android/media/routing/IMediaRouter.aidl \