Adds new magnificationController API: getCurrentMagnificationRegion, resetCurrentMagnification

See go/b200769372 - the legacy public methods
The legacy magnification controller methods of a11y service,
  getMagnificationRegion(),
  reset(),
will keep the behavior before that control
only full-screen magnification.

To make the a11y service able to control the actiavted
magnifier on the display, the service should use the
new public APIs,
 1. getCurrentMagnificationRegion(),
 2. resetCurrentMagnification(),
instead of the legacy APIs.

Bug: 210069654
Test: atest AccessibilityMagnificationTest,
   atest MagnificationProcessorTest,
   atest AbstractAccessibilityServiceConnectionTest,
Change-Id: I76bfee43cbb76a9aeec33574a8db18430903e08c