Fix race between app crash and broadcast

The original BroadcastQueue already had the mechanism to detect app
crash before actually sending the broadcast. But it still leaves a tiny
window that allows a rare race issue. The introduction of App Freezer
happens to hit this race issue if the frozen app receives an unexpected
sync binder call before a broadcast.

Fix it by checking if the app is still alive immediately after sending
the broadcast to the receiver.

Bug: 206145971
Test: verify the killed app (by am due to sync binder call) is launched
again and receive the registered broadcast.

Change-Id: I2f9edfb0d24f4d76018e3d241218d3e32fadede4