Track overridden subtypes when auto-switching due to HintLocales

[unlair]

Fixes #1409

This has to do with keyboardOptions.hintLocales (e.g., in TextFields) causing input languages (subtypes) to be automatically changed. This change tracks the subtype being overridden by a HintLocale so that it can gracefully reset when the HintLocale is no longer being applied.

If the user switches languages after a HintLocale is applied, the overridden subtype is cleared so that the new language will stick until a new HintLocale is applied again.

A possible limitation unresolved by this PR is that a HintLocale will always override when first encountered, so if you manually switch to something else and go to another app then come back, it will clear your selection.

[Helium314]

Thanks, this should be fine, also with the limitation.

I think it would make sense to store the previous subtype in SubtypeState, and maybe move the switching based on hint locales there (not sure why I hadn't put it there previously).
Do you agree, or is there a good reason to keep it the current way?

[unlair]

Thanks! I agree, would be better there. I added some changes to do that. I left the actual switching logic outside of SubtypeState and instead let the caller do the switch with the subtype returned by SubtypeState, to avoid having SubtypeState calling LatinIME.mHandler directly.

[Helium314]

Thanks!