Keyboard Shortcuts: Add Keycodes for shortcut representation

This changes the current API given app feedback received. In order to use
keycodes as well as chars we also must implement a mapping of Keycodes to
Display labels as only keys with a single char representation are provided
by KeyCharacterMap.

Bug: 27409783
Change-Id: I3da653adc4b8cbc66a4d1aa24a5f9181f71e83c3