Update TestableContext annotation from ClassRule to Rule

@Rule runs for every test method, while @ClassRule just once for the class. TestableContext needs to be cleared after each test, so @Rule should be used.

Test: Ran the test locally.
Bug: 414928899
Fix: 414928899
Flag: TEST_ONLY
Change-Id: Ie8b59f0ba537a91ada1322c49d45578e7b79bfb8