[homekit] delay starting until a particular StartLevel (#13877)

[homekit] delay starting until a particular StartLevel (#13877)

* [homekit] delay starting until a particular StartLevel

instead of a blind "delay by seconds", the new default won't even
attempt to start until items are loaded (both file-based and UI-based),
with an additional config to let the user delay it any further (in
case they have items coming from JSR223 scripts that can't run until
at least SL20 anyway).

Signed-off-by: Cody Cutrer <cody@cutrer.us>