ExtView: Add keyguard specific interface for KeyguardExternalView

KeyguardExternalView requires extra state callbacks that can be passed
on to the KeyguardExternalViewProvider.  For this reason the
KeyguardExternalViewProvider no longer extends ExternalViewProvider
and instead provides the full implementation needed for ExternalViews
as well as the specific keyguard components.

Change-Id: Icf3589a201bf0ab29ba6e00e2bd6c0149e955e1d