Updating isShowing during onResume()

Since the callback is unregistered in onPause(), KeyguardFaceUnlock
doesn't get onKeyguardVisibilityChanged() when it's paused.  This means
that if the state changes while it's paused, then mIsShowing is incorrect
when it resumes.  This is fixed by keeping track of the state in
KeyguardUpdateMonitor and checking it when onResume() is called.

Change-Id: I67aa1e091161a965e4347dc8c2ec5e7a1dd9d92e