Make DPM API for disabling keyguard widgets more generic

This change renames the widget-specific API to be more generic
to allow further disabling of keyguard-specific customizations
in the future.  Currently only allows disabling widgets and the
secure camera but can now easily be extended to disable other
features we add.

Fixes bug: 7021368

Change-Id: I3934cc2e7c64e0c6d511efb86980fc38a849708d