Fix race condition in WindowManagerService.disableKeyguard()

TokenWatcher.acquire() synchronizes on mTokens, not this,
so we need to synchronize on mKeyguardDisabled in disableKeyguard()
to synchronize properly with reenableKeyguard().

This should fix b/2180142 (Stuck in enable keyguard when receiving phone call)

Change-Id: Iad66a2748c7fbf2c516fdb8a00988696719ea80c
Signed-off-by: Mike Lockwood <lockwood@android.com>