Make AccessibilityModule optional.

Wear has its different implementation of Magnification border. So this
change does following things to unblock wear customization:

1. Change Magnification from a class to interface to allow wear creating
   alternative implementaiton.
2. Make AccessibilityModule part of ReferenceSystemUIModule instead of
   the base SystemUIModule. This allows wear to insert its own
   accessibility module without inheriting the default module.

Flag: EXEMPT refactor
Bug: 331679109
Test: build success + manual test
Test: atest MagnificationTest
Change-Id: I795a1cef0513f7f5f929eb192f49ef10c8502e65