Stop all test apps after every test run

This CL changes ActivityManagerProcessStateTest to forcefully stop all test apps after every test run.
Otherwise, if another test, which uses the same test app, runs after the test sequentially, the following test gets interfered with by the ActivityManagerProcessStateTest.

Bug: 261798498
Test: Run ActivityManagerTest after running ActivityManagerProcessStateTest
Change-Id: I1dcbcf83a4c4f734c0f4a72aa252b7f44c70f862