TrustManagerServiceTest: Fix broken tests

These tests currently fail because of missing setup for
ActivityManagerInternal.class in LocalServices. Set it up
with a mocked instance, and clean up properly.

Bug: b/323086607
Test: `atest TrustManagerServiceTest`
Flag: TEST_ONLY
Change-Id: Idb92137c2fbc8a50490b7cd1813670685f9a969d