Refactor DPMS Cert Installer and App Restrictions delegation.

The DevicePolicyManagerService currently supports delegation of
certificate installation and application restriction management, both
of which are individually handled by DPMS.

Upcoming framework features will add four more delegation types,
namely: block uninstall; app permission management; app access
management; and system app enabler. At this moment it makes sense to
refactor the underlying delegation system in DPMS so that current and
future delegates can be handled in a more generic way.

Bug: 33099995
Test: DPMS unit tests
Change-Id: I9e350143572c6690febdd59d1ed5149af8ee4388