R1:c4de87a07ab0

Improve some settings-related performance

Summary:
Ref T4103. Two small improvements:

- Don't work as hard to validate translations. We just need to know if a translation exists, we don't need to count how many strings it has and build the entire menu.
- Allow `getUserSetting()` to work on any setting without doing all the application/visibility checks. It's OK for code to look at, say, your "Conpherence Notifications" setting even if that application is not installed for you.

Test Plan: Used XHProf…