Interface with Mouse keys A11Y feature flag and Setting

Add the APIs to check whether mouse keys feature is enabled and to set
the mouse keys to be enabled if it is turned on via Settings.

DD: go/mouse-keys-android
Bug: 341799888
Test: Presubmit
Flag: com.android.hardware.input.keyboard_a11y_mouse_keys
Change-Id: I33dc19c451973f87a4a7c34da3863d0ad54f0488