Backport of ordering apps for boot dexopt.

This is a squashed commit of the following changes:

1. Order apps by priority when performing boot dexopt.
   (cherry picked from commit 65cde7d4)

2. Improve priority ordering of apps when performing boot dexopt.
   Added core apps and updated system apps.
   (cherry picked from commit 272bf3a2)

3. Stop boot dexopt when low on memory.
   (cherry picked from commit 1d892dcb)

Bug: 17641843
Change-Id: Ie32f1c21047d3462aaf728f7633fecf647ba2b47