Keyguard: Third party keyguard support

Enables support for third party keyguards.  Currently for use with
the themable animated lock screen.

This is currently only for system apps or apps signed with the same
key as the system.

issue-id: YAM-115
Change-Id: Ifcbf9368d951da3ea7ab8e6205d08acb078d8e5d
(cherry picked from commit bbc89853)