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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:adaa383860c2a6de6c6ecb77a35dcd9909779e4b)
Merged-In: Idb92137c2fbc8a50490b7cd1813670685f9a969d
Change-Id: Idb92137c2fbc8a50490b7cd1813670685f9a969d