Refactor shortcut handling into before/after key capture stages

With key capture APIs, we need to split shortcut and system keys
handling into stages:
- Before key capture: System keys like functional row and
  non-capturable shortcuts (Home + Desktop windowing)
- After key capture: All remaining system shortcuts that only get
  executed if the focusing window doesn't have key capture enabled
- Unhandled shortcuts: Shortcuts that need to be executed if key
  capturing window doesn;t consume certain keys

Test: atest InputTests
DD: go/key_capture
Bug: 416681006
Flag: EXEMPT refactor
Change-Id: Ia3f8e63a6ac69b60ac100aed8b844e2e2629b572