Refactor key handling in interceptKeyBeforeDispatching

Moving the logic from the if-else statements to a switch-case statement,
that could help us to maintain the key handling easier.

This also extracts below shortcuts into switch-case statement :
META-ALT : toggle caps lock.
ALT-TAB : display task switcher.
CTRL-SPACE/META-SPACE : keyboard layout switching.

Bug: 162194917
Bug: 127687575
Test: manual with connected keyboard
Change-Id: I1e392ac98135150421e2ea26daeb2a7b7821a618