Implement magnification toggle keyboard shortcut

Adds new "KEY_GESTURE" shortcut type to the AccessibilityManagerService
which tracks Accessibility Services which are able to be activated
directly from a KeyGestureEvent key combo.

Bug: 375277034
Test: atest AccessibilityManagerServiceTest
Test: Manually on device
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures

Change-Id: I4d65eb7834d1b48628fca46802781054901c0287