Avoid deadlock during device boot up

PowerManager invokes registerReceiver while holding mLock
when system ready. There have chances that the system_server
main thread will be blocked to wait for ActivityManagerService
object while the other thread was performing configuration
updates.

Solution:
Avoid holding locks while invokes registerReceiver()

Similar issue:
https://code.google.com/p/android/issues/detail?id=186872

Change-Id: I6f3db0758959bef5d8872c74e28e0491ee786ee6