Fixed delay of thumbnail loading when going to overview from home

Some thumbnail snapshots show up slowly when GRID_ONLY_OVERVIEW is enabled because more thumbnails are displayed than before and some cache related code needs to be changed. This issue should be fixed for smooth user experience so all thumbnails could be shown quickly and at the same time.
Root causes and corresponding solution are as below:
1. No high resolution thumbnails are in cache when entering overview. It is either no cache at all or just low resolution cache
Solution: Preload when high resolution is enabled
2. The cache size is 3 which is not enough for tablet
Solution: Update cache size to 8 when it is tablet, and preload more if cache size increases. Remove least recent ones when cache size decreases

Fix: 271570416
Test: RecentsModelTest
Test: TaskKeyByLastActiveTimeCache
Test: TaskThumbnailCacheTest
Change-Id: I60003ca20942c256a0a1047678ea4368cd2520eb