chore(magnification): add magnification sysui connection state changed callback for cts tests

In accessibility magnification cts tests we need to ensure the
magnification system ui connection is ready for the coming tests.
Therefore in AccessibilityService#dispatchServiceConnected we add a
conditional magnification sysui connection state check, so if the
service find itself has capability to control magnification, it will
wait until the magnification sysui connection is ready then trigger
onServiceConnected callback.

The fix is gated with a bug-fix flag so we can verify it with rollout
process.

Bug: 337800504
Flag: ACONFIG android.view.accessibility.wait_magnification_system_ui_connection_to_notify_service_connected DEVELOPMENT
Test: pass build
      atest AccessibilityMagnificationTest
Change-Id: I3e38ba32515eadd8f5eac6aa5fd906eeb25e24c9