PackageManager: Add option for single threaded dexopt

* Execute all dexopt invocations on the main thread instead of using the
  ExecutorService when persist.sys.dalvik.multithread is set to false.

Change-Id: I3b8499d5af5091ad7abcddf006f8f2837943903b

Conflicts:
	services/java/com/android/server/pm/PackageManagerService.java