Support A11y contextual button

Also migrate to only using sysui state flags
for ime/switcher visibility instead of 2 separate
methods.

Bug: 180046394
Test: Tested w/ 1 a11y service, then 2 services for
long click. Tested with IME switcher enabled and then
also suggested rotation button.
A11y takes precedence over IME switcher, but rotation button
takes precedence over a11y as is in phone mode today.

Change-Id: I9289165c8ca98d7ee432bd7145122d6a519600f6