Add tests for MagnificationController.

Also refactoring the class to make it easier to test and
chaning behavior where the current behavior seemed poorly
defined.

Refactoring:
- Combined all handlers into one.
- Simplified animation to use a ValueAnimator.
- Eliminated ACCESSIBILITY_DISPLAY_MAGNIFICATION_AUTO_UPDATE
  setting. Move rest of settings reading into mockable class.
- Move callbacks from WindowManager into the main class.
- Pulled out my instrumented Handler from the
  MotionEventInjectorTest into its own class so I can reuse
  it.

Behavior changes:
- Always constraining out-of-bounds values rather than
  refusing to change them.
- Constraining offsets on bounds changes. We previously
  left them alone, even if they were out of bounds.
- Keeping track of the animation starting point. We were
  interpolating between the current magnification spec
  and the final one. This change means the magnification
  animates to a different profile.

Test: This CL adds tests. I've also run a11y CTS.

Bugs: 31855954, 30325691

Change-Id: Ie00e29ae88b75d9fe1016f9d107257c9cf6425bb