The legacy magnification controller APIs controls only full-screen magnification

See go/b200769372 - the legacy public methods
The legacy magnification controller methods,
  setScale(), setCenter(),
  getScale(), getCenterX(), getCenterY(),
  getMagnificationRegion(),
  reset(),
will keep the behavior before that control
only full-screen magnification.

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

This change
1. Updates the doc of the legcacy APIs.
2. The legacy APIs controls only fullscreen magnification

TODO: Add new public methods, getCurrentMagnificationRegion(),
 resetCurrentMagnification() for the current controlling magnifier

Bug: 210069654
Test: atest MagnificationProcessorTest,
    atest AbstractAccessibilityServiceConnectionTest,
    atest AccessibilityMagnificationTest,
Change-Id: Ia157971d2c9716df7aa14024525ee0adfdc0d46f