Support tappable elements on left/right sides

This is to let window provide tappable elements on left or right side.
This is useful when there's a task bar. The task bar needs to provide
tappable elements insets on the side where the bar is, and it may not be
the top or bottom side.

Bug: 172289399
Test: atest InsetsStateTest
Test: go/wm-smoke
Change-Id: I5bc8d010f4ad212acb779265aff1d2622f5e2da4