Shortcut target migration and automated button mode switching

When switching from gesture navigation to navBar navigation,
All gesture targets are migrated to the software target list.
When switching from navBar to gesture, software targets are unchanged.

A11y button mode is changed in certain conditions
When in gesture navigation mode, button mode is always set to FAB.
When in navBar navigation mode, and there are >0 gesture targets and 0 software targets,
button mode is set to navBar.
There is no condition in which software targets are migrated to the gesture target list.

Test: atest AccessibilityManagerServiceTest, and verify behavior manually
Bug: 352165471
Flag: android.provider.a11y_standalone_gesture_enabled
Change-Id: I6adf818b532d5fb1951dd5588eccb446b3bd4783