Refactored TrustAgentUtils to allow agent check injection.

Notes:
- TrustAgentUtils defined a method that allowed callers to determine if a Trust Agent package is allowed to provide trust. If it is, it adds it to the list of available trust agents that can be displayed in the Settings->Security screen.
- The logic used to define what agents are permitted on the device can now be injected.

Test: Ran 'm RunSettingsRoboTest' and added a new test for the TrustAgentFeatureProviderImpl.
Bug: 34354635
Change-Id: I24c54c14bde26073ce6fa907379b86aae2841600