Add keyguard background scrim and protection around keyguard APIs

With this change, the system process will put up a scrim in the
event keyguard crashes to protect underlying content.

It also adds permission checks to prevent unathorized access
through the binder APIs.

Cleaned up KeyguardTestActivity to build separately.

Removed unused resources.

Change-Id: I9e370c6bfb7dca68eae9eae304c815fb84a753d2