Clean up SystemService as a preparation to make @SystemApi

Bug: 146145144
Test: Boot + treehugger
Test: atest CtsAutoFillServiceTestCases (with some unrelated failures)
Test: atest CtsContentCaptureServiceTestCases CtsVoiceInteractionTestCases (all passed)

Change-Id: I89265c3c46262cd3251153f85df6f4aea37520f1
6 files changed