ActiveLocale implementation is probably larger than it needs to be, not cross-platform, buggy

[bzbarsky-apple]

Problem

The ActiveLocale implementation from #13415 has the following problems:

1. Lots of support code used that shouldn't be needed.
2. Requires per-platform implementation for what should be cross-platform functionality.
3. Does not implement the spec correctly. It's missing this bit:

An attempt to write a value to ActiveLocale that is not present in SupportedLocales SHALL result in a CONSTRAINT_ERROR error.

Proposed Solution

1. Instead of implementing via configuration manager and attribute access override, use NVM attribute storage, which works on all platforms without requiring per-platform implementations.
2. Use a PreAttributeChangedCallback that reads the SupportedLocales list to implement the spec's CONTRAINT_ERROR bit.