We now only keep a thumbnail for the task, not for each
activity. However if you use FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET,
we will make a new secondary thumbnail for that series of
activities. There is a new API for the app to get these
secondary thumbnails.
Also set a default thumbnail size for non-xlarge screens
so we have thumbnails on phones. (We need some smarter
code in the platform for computing the actual thumbnail
dimensions of the current device). And add a test app
to show recent tasks + thumbnails.
Change-Id: Ic36759f6635522118a2cb7f156662229a610c492
