Refactor magnification keyboard control to new gantry flag

This moves magnification keyboard pan and zoom to be under
the flag
com.android.server.accessibility.enable_magnification_keyboard_control
instead of
com.android.hardware.input.enable_talkback_and_magnifier_key_gestures.

Both flags are at staging so there is no change in functionality
on most platforms. Overrides new aconfig flag on desktop so that
there is no functionality change there, in separate commit.

Bug: 388847194
Test: Existing
Flag: com.android.server.accessibility.enable_magnification_keyboard_control
Change-Id: I2d800e084b03a8cad5e1b607dc424d3aa7f13975