[1/2] Speed up booting on multicore devices.

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

Change-Id: Id42b2bf4a737f93f9ecd9b5819344adbb735f0fb
Merge "[1/2] Speed up booting on multicore devices." into kk4.4