Aggressive tuning of recents memory usage

Limit the thumbnail count to a maximum of 10 thumbnails and the app
icon count to a maximum of 20 thumbnails, instead of using a
byte-size based limit.

Also prune entries immediately when leaving recents.

Bug: 16513124
Change-Id: Id9a32f87ca3f9f19e5cad5f115d54b19b26f4f93