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>

services/java/com/android/server/WindowManagerService.java

@@ -3971,7 +3971,9 @@ public class WindowManagerService extends IWindowManager.Stub
             != PackageManager.PERMISSION_GRANTED) {
             throw new SecurityException("Requires DISABLE_KEYGUARD permission");
         }
-        mKeyguardDisabled.acquire(token, tag);
+        synchronized (mKeyguardDisabled) {
+            mKeyguardDisabled.acquire(token, tag);
+        }
     }
 
     public void reenableKeyguard(IBinder token) {