Fix and improve RequestFocusTest

RequestFocusTest used the deprecated ActivityInstrumentationTestCase2
and associated support. It also uses the @UiThreadTest annotation which
is only supported in junit4 leading to half of the suite failing with
exceptions due to calls on a non-main thread.

This change:
- migrates to junit4
- uses modern constructs like ActivityScenarioRule
- moves UI thread dependent logic to ActivityScenarioRule.onActivity
- resolves new base assumption of default touch mode by adding logic to force non-touch by sending a key event
- rebaselines behaviors e.g. global focus change is no longer called
- motivates more tests for keyboard + focus
- will be used to also adapt this suite for system + a11y focus testing

Bug: 393668723
Test: atest RequestFocusTest
Change-Id: Ifcdb1c5a5bb7508ed94ee4799f1412d13bab6086