Fix system action API issues in SystemActions

1) Actions are only registered when the service starts. If the locale
changes, the labels need to be translated, so re-register the actions
when this happens.

2) When its shortcut isn't visible the a11y menu action is a no-op. The
list should be an accurate representation of what actions are available.
Pass SystemActions into NavigationBarFragment, which keeps track of the
shortcut availability,  to unregister/register the action.

To do this, SystemActions exposes register/unregister to the fragment
and makes SYSTEM_ACTION_ID_ACCESSIBILITY_MENU public.

3) Remove Toggle Split Screen from SystemActions. The trigger isn't
system-level but is rather buried in the Recents UI in a button for each
app, and the a11y user can access this like everyone else.
SystemActionPerformer will still handle the legacy action call in
performSystemAction.

4) Rename "Accessibility Menu" to "On-screen Accessibility Shortcut"

Bug: 152636060, 152635646, 154833492
Test: Manual TalkBack test
Change-Id: I9b037f91c8d3b6f193fc9aee95ef73b7f3fbf315