Use cached thumbnails in Recent tasks.

The thumbnail returned from ActivityManager.getTaskThumbnail() now
contains either a Bitmap or a ParcelFileDescriptor that points to
a file containing a compressed Bitmap. The Recent tasks list is
now responsible for all thumbnail Bitmap caching as the activity
manager keeps only the most recent 5. This also permits low memory
devices to have many more tasks in the Recent tasks list.

As part of this CL the concept of subtasks is removed eliminating
code supporting the TaskAccessInfo and IThumbnailRetriever classes.

Fixes bug 15828934.

Change-Id: I0fd0320a1a04e3c78d79357899b83a2fff97abf2