Add API to register listener for Sticky modifier state

When Sticky keys is enabled, this API will provide a way to SysUI
to listen to changes to modifier state and show some UI,
that repesents the sticky modifier state.
DD: go/pk_accessibility

Test: atest StickyModifierStateListenerTest
Bug: 294546335
Change-Id: I789c17ca09d47dae279f4005954cfeea7928ed3f