~Putting this up as draft for now, gonna see if I can do a dual test
insertion to confirm this makes RPS happy. The UI context is defined in
dotnet/razor#12079 so its a little bit painful
to validate everything together :)~

This PR does two things:
1. Makes the lifetime service Lazy, which it should have always been but
I forgot. Not being lazy means Razor can't move to it, as just MEF
construction causes RPS regressions. Oops!
2. Adds a ui context to control pre-initialization. This allows Razor to
control whether that occurs, when, and whether it actually does
anything.

Until Razor is in this won't do anything, but runs in the dual test
insertion
(https://devdiv.visualstudio.com/DevDiv/_git/VS/pullrequest/664449) look
good so far.