[A11y] Move Magnification key handling to AccessibilityInputFilter

This change has no functional difference but moves magnification scale
and pan key press handling from KeyGestures to AccessibilityInputFilter.
In follow-up changes, this will allow Magnification users to hold the
magnification keys in order to continuously pan/scale the viewport,
and will allow handling of diagonal panning.

The magnification scale and pan keyboard shortcuts are moved to the
InputGestureManager blocklist so that they cannot be clobbered by
system shortcuts.

Bug: 388847194
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Test: atest MagnificationKeyHandlerTest, AccessibilityInputFilterTest, and manual
Change-Id: I5a14908052d397b0b372d324d1971a3caec49ad0