Improve TouchDelegate Accessibility: Explore by Touch handle hover events

- Add TouchDelegate#onHoverEvent

Bug: 35702820
Test: Install TestBack with flag FLAG_REQUEST_TOUCH_EXPLORATION_MODE
      enabled. In Settings APP, enable TestBack then touch/hover into
      most left side of SwitchBar in the same subactivity to confirm
      Switch delegated and hover enter event exist
Test: manually test with sample app in issue #7. Touch button delegate
      and hover move over text view to confirm text get a11y focused.

Change-Id: I0abea81ea2fee4d391e2ee448710c5f0180f7533