Use target context in process tests

Use test target context instead of instrumentation context to launch
activities from the same package. Otherwise, the activity launch is
restricted as one that is coming from wrong UID.

Bug: 148421915
Bug: 148422846
Test: ActivityManagerProcessStateTest
Test: ActivityManagerTest
Change-Id: Ifb0eb8735fdf8a029dbf1b3ca94bf2468bfafa26