Explicit UiThreadTest, Rule to block @UiThreadTest

The @UiThreadTest annotation wasn't working with AndroidJUnit, because AndroidJUnit4ClassRunner was replacing with FailOnTimeout.  This rule will assert on the use of @UiThreadTest, and a utility to execute runnables on the UI thread explcitly, with associated tests.

Bug: 369878945
Test: tested manually with atest
Flag: TEST_ONLY
Change-Id: I583bb0878b1dfe21fb17eec30dc600026ba482f4