Adjust taskbar VIS and bottom mandatory gesture insets

- ITYPE_BOTTOM_MANDATORY_GESTURES should always be > 0  in gesture
  navigation mode, but before we were setting it equal to tappableHeight
  which is 0 if the taskbar is stashed. Fixed it to be contentHeight
  instead.
- Updated VIS insets to only override them to be 0 for tappableElement,
  not for others like navigationBars. We still only override this for
  gesture navigation; 3 button mode will continue reporting the same as
  we report to the underlying apps.

Test: manually with 3 button mode, gesture nav mode with taskbar
stashed/unstashed; checked that TaskbarInsetsController dump looks as
expected, and that behaviors like Assistant and IME look correct
Flag: none
Fixes: 262516176

Change-Id: I5a1cdf5a9956777d3189ef6042493e2f5aa9b162