Add experiment to skip creating UIViews altogether for constants

[Adlai-Holler]

This also fixes a missing exp_unfair_lock constant 😅 .

This can avoid some pretty serious pre-main time.

The diff is hard to read, so here's what it does:

• Create static NSNumber *'s for the two BOOLs. If you're not in the experiment, these will be set during ASInitializeFrameworkMainThread. If you are in the experiment, the initializer function does nothing.
• In the two accessor functions e.g. BOOL ASDefaultAllowsGroupOpacity() we have a dispatch_once that checks the static to see if it was set in the initialize function, and if not it selects the default based on UIKit documentation (by reading Info.plist).
• There was some thread-unsafety before which is maintained. Namely, the statics were not thread-affined and they still aren't but the dispatch_once has a memory barrier which should make us safer even than before.

[maicki]

LGTM