Ensure surfaces stay alive until activity stop.

Prior to this commit in this case of activity pause, with finishing=true
the activity manager will notify us of app visibility and we will begin
an exit animation. When this exit animation finishes, we will destroy
the application surface (unless its eligible for saving). However there
are two cases where this breaks down:

1. The exit animation finishes before the activity thread handles
the stop transition. Many activities stop rendering on Pause
but many do not and it is totally legal to do so. Sometimes this
results in non fatal dequeue buffer errors and sometimes results in
fatal errors with Pixel Buffers, etc...
2. We may resume the activity shortly after asking the window manager
to pause it. If the window wasn't eligible for animation, we will
immediately destroy it after being told of the visibility change
following PAUSE_FINISHING. It's possible for this to complete
before we process the resume. On the other hand the client
happilly processes the resume and transitions back from PAUSE
and then crashes once it attempts to use it's surface.

In this commit we have the activity manager notify the window manager
when an application has actually finished (or we have timed out
waiting). For windows which have not been explicitly removed by the
client, we defer destruction until we have received both this signal
and the animation has completed.

Bug: 26793431
Change-Id: Ib6ee8fbdd1f03450fbbfd62468a19e97774befab

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