Implement magnification viewport pan shortcuts.

Adds keyboard shortcuts to pan around both the fullscreen and
windowed magnification depending on which is currently in use.

Shortcut keys are not officially decided and are subject
to change.

Bug: 355499907
Test: `atest MagnificationControllerTest` + locally on device
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Change-Id: I72e88290e4b5921e229f02f04d18aac11f303f4c