Listening to setting changes in StickyKeyRepository

Implementing settingEnabled: Flow<Boolean> that listens to changes in
Settings.Secure.ACCESSIBILITY_STICKY_KEYS which means sticky key listener will be
registered only when user turns on the setting - compared to now listening always when
keyboard is connected. Also supporting per-user setting.

Test: StickyKeysIndicatorViewModelTest
Test: StickyKeysRepositoryImplTest
Test: switch between users having enabled/disabled sticky key setting and see if behaviour is correct
Flag: ACONFIG com.android.hardware.input.keyboard_a11y_sticky_keys_flag TRUNKFOOD
Fixes: 319837892
Change-Id: Iebe5d6aa38e633a81d38a0fb95645a91ed2575b7