Switch to showing top-most thumbnail of recent apps.

The way it should have been, and with the new recents enter animation
the way it must be.

Added a new method to retrieve this thumbnail, since it would be less
efficient to use the existing API (which always returns the "base"
thumbnail).  Probably at some point that existing API should be tweaked
to always return the top thumbnail instead, but that is for a later time.

Also removed code that would clear the thumbnail associated with an
activity when it is resumed.  I don't think there should ever be a
reason to clear a thumbnail -- it's much better to have *something*
for the task, even if it is a little out of date.

Change-Id: I83e6ca6403eb2df5e4de3009dfe8c210e8cf8d5b