Adds public APIs for AccessibilityService to control magnifier by setting MagnificationConfig

See go/b200769372
AccessibilityService adds new magnificationController APIs,
setMagnificationConfig and getMagnificationConfig, to control
magnification on demand.
The service can set the magnification mode, scale, center postion
of the magnifier on the display to control magnification.
And the service controls the least recently activated magnifier
on the display if the config mode is not specified.

Bug: 199732498
Test: atest MagnificationProcessorTest,
    atest  AbstractAccessibilityServiceConnectionTest,
    atest AccessibilityMagnificationTest,
Change-Id: I51be2bbf96411c6c54c9da30873556d09533bb65