Fixed some small keyboard nav bugs (TabHost, directional focus)

- TabHost was forwarding Tab/shift+tab/space to its content
  instead of navigating through the tabs.
- FocusFinder's directional focus considered 2 touching Views to be
  overlapping which would yield confusing directional movement at
  times.

Bug: 62943663
Bug: 62911028
Test: Added CTS tests
Change-Id: I253f66b7513daf5c2c9fdeed8a2fb930b4fd8db4