Input: add option to make 3-finger tap a shortcut

By default, it will remain as middle-click, but if the user chooses to
customize it then the native code will make a JNI call to
KeyGestureController via InputManagerService. The KeyGestureController
implementation to actually treat it like a shortcut will be implemented
later.

Test: 3-finger tap with flag enabled, check log line appears
Test: 3-finger tap with flag disabled, check button events appear at
    https://w3c.github.io/uievents/tools/mouse-event-viewer.html
Test: atest com.android.server.input.InputManagerServiceTests
Flag: com.android.hardware.input.touchpad_three_finger_tap_shortcut
Bug: 365063048
Change-Id: I6ba5e2235c7429db173b502df19fc40b28822c0f