Cancel or drop key events if activity is stopped.

After an activity instance state is saved due to onStop()/onPause(),
there is a small window where it can still get key events like the
back button since we still allow the ActivityThread to handle
pending messages (like memory trim request) before informing the
activity manager that we are done. If the activity is stopped,
we will now drop non-terminal input events and set the cancel
flag on terminal events.

Bug: 18151331
Change-Id: I370d7c871530eea4b16fa42428d0248f1a87abb6