init: refactor keychord for testing

Move things around so that keychords.cpp is independent of service
and init and can be individually tested with few dependencies.
Permits also rolling out the keychords as a class in a future commit.
Improve parser checking.

Test: init_tests
Bug: 64114943
Change-Id: I82570bc6269ed478db784ec38a8bc916da2be2be