Speed up booting on multicore devices [1/2]

1. Add multithreading support to Installer/installd.
2. Parallelize package scanning.
3. Parallelize boot dexopt.

Change-Id: Id42b2bf4a737f93f9ecd9b5819344adbb735f0fb
Merge "Speed up booting on multicore devices [1/2]" into du44