fix TrustAgentsPreferenceControllerTest

Removed non-platform Robolectric shadow in favor of setting up fake
package/services directly with ShadowApplicationPackageManager.

Bug: 313612480
Test: atest "SettingsRoboTests:TrustAgentsPreferenceControllerTest" --host
Flag: TEST_ONLY
Change-Id: Iadcb5bcb977201e2f394b699621bfb34657a7820