a11y: Add first-time use dialog for key gestures (3/n)

This patch provides
* Implemented the ui layer
* Implemented the unittest for the startable

Bug: b/410892855
Flag:com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Test: atest KeyGestureDialogStartableTest

Change-Id: I6e388d0fd31ec8933ad765771c666d43fc90f8cd