Refactor KeyDebounce class

This change moves the following methods into KeyDebounce class.
- MotionEvent handling methods (onDownEvent etc.).
- Keyboard action event generator method (detectAndSendKey).
- Multi-tap key support methods (resetMultiTap etc.).
- Key preview text generator method (getPreviewText).

The change that makes KeyDebounce a top-level class will follow.

Bug: 2910379
Change-Id: I1526c0461e247d97f0c4b3bd580330f137c787be