[A11y] Update magnification panning keyboard bindings

After an email discussion, it has been decidd that split screen
focus changing keyboard keys will be removed, and magnification
can have action + alt + arrows to be consistent with magnification
zooming with action + alt + plus/minus.

This change updates split screen focus keyboard keys since they
are not yet deprecated to ctrl + alt + left/right arrow, and
sets the magnification keys to the agreed-upon combination.

In addition, renames "magnifier" to "magnification" to avoid
confusion with the Pixel magnifier app, separate from
magnification.

Bug: 384514582
Test: `atest KeyGestureControllerTests` and manual on device
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Change-Id: I583bc604cd78c60c681ec0c5ef499b0afadb199c