Introduce SystemLocaleWrapper

As part of preparations to support concurrent multi-user in IMMS, this
commit introduces

  SystemLocaleWrapper,

which takes care of system locale access in general around IMMS.

With this CL, you can get LocaleList for a given user with

  SystemLocaleWrapper.get(userId)

rather than

  mRes.getConfiguration().getLocales().

Bug: 309837937
Test: presubmit
Change-Id: Iafbba77f4069121f2efd5ac6a2fb23499dd4503d