Extract a11y from NavigationBar to share with TaskbarDelegate

When taskbar is showing, navigation bar isn't created which
means all of the a11y registers are never registered.
NavigationBarA11yHelper maintains existing a11y flag logic from
NavigationBar but any class can register with it to get updates.

Bug: 180046394
Test: A11y works as expected on 3 button phone
Right events getting pass through to launcher when on larger
device

Change-Id: I9eeb566fca0e5142b38127aa8f32c21632c33caa