GestureConverter: 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 we'll handle it like a special keyboard shortcut by sending
a JNI call up to InputManagerService.

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 inputflinger_test:GestureConverterTest
Test: m checkinput
Flag: com.android.hardware.input.touchpad_three_finger_tap_shortcut
Bug: 365063048
Change-Id: I771d46e1aafe9164dd32771cfdb55a3b259b5a0d