Fix nav bar consuming

- Cache shouldAlwaysConsumeNavbar so it doesn't get reset when
insets == null
- Remove logic with frame comparison when determining whether to
consume nav bar. Not sure how that ever worked.
- Make sure shouldAlwaysConsumeNavBar survives when consuming
insets.

Bug: 27157904
Change-Id: I35f209ab27cc12240038da7efa9e79c95f70c6ce