Implement magnification scale control shortcuts

Adds keyboard shortcuts to zoom in and out of both the fullscreen and
windowed magnifier depending on which magnifier is currently in use.

Bug: 375277090
Test: atest + locally on device
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures

Change-Id: Ia5a02168c03ac1f40fc4e326bdd10f393fecce14