Auto enable new system trust agents

Previously we only auto enable factory system trust agents when the
devices is booted for the first time. This change added logic to detect
newly installed/enabled trust agents, and enable them if appropriate.

Fixes: b/243705470
Test: atest FrameworksMockingServicesTests:TrustManagerServiceTest
Change-Id: I3c345a44cfcfccc6f7e2507ba37e378b6b16bd98