Fix deadlock in WindowManagerService.reenableKeyguard()

If reenableKeyguard() is called before the previous disableKeyguard() call is processed,
then TokenWatcher.sendNotificationLocked() will cancel the request, resulting in neither
the TokenWatcher acquired() or released() methods being called.
In that case, reenableKeyguard() will hang waiting for released() to set
mWaitingUntilKeyguardReenabled to false.  Now we only wait in reenableKeyguard()
if the TokenWatcher acquired() method is called and the keyguard has actually been disabled.

This should fix bug b/2270192

Change-Id: Id886fb28df607dbb4543124f2db6997121d6a682
Signed-off-by: Mike Lockwood <lockwood@android.com>