Reduce cost of a11y services with magnification.

Services that declare that they can control magnification,
but never actually make a change or register a listener
waste cycles as we compute magnification data they never use.

Avoid registering for magnification callbacks unless magnification
gestures are enabled, a service is listening for magnification
changes, or a service has changed magnification.

Bug: 28425922
Change-Id: I114a833669bd53b2cd757c94ea52b65a2f838a08