Refactoring magnification controller for accessibility service connection

It is preliminary change for AccessibilityService to control window
magnifier.
To make A11y service connection able to control magnification,
a11y service connection has to use the magnification controller
instance.
However, there will be multiple magnification mode for a11y service to
control. So we add magnificationProcessor class that wrap the common
functions of the magnification controller to make the systemSupport
interfaces less redundant.

Doc: go/a11yservice_control_magnification_in_t

Bug: 197205358
Test: atest AbstractAccessibilityServiceConnectionTest
 atest AccessibilityInputFilterTest, atest
 AccessibilityServiceConnectionTest, atest MagnificationProcessorTest,
 atest android.accessibilityservice.cts.AccessibilityMagnificationTest
Change-Id: I41f0a56b5545711498eaf6383794736711db1f9c