Refactored the contextual group of buttons that appear on the nav bar

Refactored NavigationBarView's contextual buttons to keep track of just
one button's visiblity with its container group. The visiblity is
determined by the priority of the buttons. For example, accessibility
has the highest priority, if that is visible, no other buttons can take
its place, however if menu button is visible, then rotation can appear
instead.

Bug: 116041410
Test: atest NavigationBarContextTest
Change-Id: Ic0762c7b7121a313b7c08989f9ab761426372c5c