Add touch feedback to links

- Clear selection after LinkSpan is clicked so that the highlight
  effect will be cleared when the tap completes
- Set focusableInTouchMode to true and revealOnFocusHint to false
  in RichTextView for N MR1 or above to allow the highlight effect
  to be visible in touch mode.

Test: ./gradlew test connectedAndroidTest
Bug: 73350031
Change-Id: Ibb6f67102775802cdfebaa1529c09d936b4096cb