Prevent ANR if window is stopped

When an activity is stopped, there could still be some pending events in
the inputconsumer due to batching. Those events are supposed to be
handled when choreographer's 'CALLBACK_INPUT' occurs, but this will
never happen if the window is stopped.

There are two possible ways these events can occur:

1. The last input event happens before onStop, but at that point,
choreographer will already stop sending callbacks. The solution to this
is to add a call to "consumeImmediately" inside setWindowStopped.

2. Input events come in after window has already been stopped. At that
point, the callback we posted in 1. would already have executed (and
found that there's no events). The solution to this is to immediately
consume all input if mStopped=true. We do this by switching to
"unbuffered" mode for simplicity.

This is reproducible via monkey testing, but it's plausible that such
race condition can occur in real world usage.

Test: adb shell monkey --throttle 40 50000
Bug: 159239700
Change-Id: I74d8f55503ef7df2fd01931afb362d68e5026e86