Improve wakelock debugging:

Throw NullPointerException if tag is null in PowerManager.newWakelock()
Print wakelock owner's uid and pid in dumpsys power output.

BUG: 2522675

Change-Id: I462c7f8c49f9896b2f58cef5a678ebd2c062a8f7
Signed-off-by: Mike Lockwood <lockwood@android.com>