Fix synchronization problem in PowerManagerService.dump()

Fixes a race condition that resulted in an IndexOutOfBoundsException
due to a wakelock being released while dump() was iterating through the
wakelock list.

BUG: 2469910

Change-Id: I10d7a65b9f397934942b290cb047c4ca81214297
Signed-off-by: Mike Lockwood <lockwood@android.com>