Refactor MagnificationController out to its own class

First step in moving ownership to AccessibilityServiceManager so that we
can implement the magnification control APIs.

The magnification Region and the current/sent MagnificationSpecs are now
private to the controller, and all access is handled through accessor
methods. Comments have been added to all public accessor methods.

Bug: 22718911
Change-Id: Ia50ee93513785585aac5424543fc1d730b84bd90