Package manager optimizations.

Addresses:
Issue #2550648: PackageManagerService setComponentEnabledSetting unconditionally
writes Settings xml
Issue #2549084: Make PackageManager.addPermission have async version

Also make the writing of settings when changing the preferred activities to use
the same async mechanism, and fiddle with thread priorities in the background
thread to go up to foreground priority when holding the lock to write settings
and a few other places.  (At some point we should really clean this up to never
acquire the main lock while in the background.)

Change-Id: Ib2b7632543f6fb3f92a225518579f3b2d15e1413