Tear down the Agent after each test.

Tear down the Agent after each test so that it doesn't continue to
operate on the Handler thread.

Bug: 299645803
Test: atest --rerun-until-failure 50 com.android.server.tare.AgentTest
Change-Id: I07f4742a1a438ce61919bf5d75a75484fb3815db
2 files changed