Add description of regional preference

  - In Andorid U, regional preferfence use language tags to record user preference of temperature unit, first day of week and numbering system.

  - ACTION_LOCALE_CHANGED is a existed action and when appending the langauge tags to locale, system shall also send out this intent to notify whole android process which registered this action.

Bug: b/270670680
Test: Make passed
Change-Id: I67e5c08c8f89989b661110760080dc53430242bb