Overlap insets with item margins

The system insets already create visual space for us, so there is no
need to add additional margin if we already have the insets acting as a
buffer. So we can just overlap them, preventing scenarios where we have
too much scrollable white space.

As a bonus, we get a more robust way of determining the clear all
button's bottom margins as we now base it directly off whether the
insets cover the space as opposed to using the orientation (which is
an indirect way of knowing the insets).

Bug: 131636735
Test: Test on 320x569 mdpi. Matches UX spec.
Change-Id: I6a3a3a5475ed303ed21bca3ed3cc363276c6e745