1. Use flag to control the UI migration
2. Create language_and_region_settings.xml for new UI migration.
Bug: 379962955
Flag: com.android.settings.flags.regional_preferences_api_enabled
Test: check hsv and atest LanguagePreferenceControllerTest
Change-Id: If8d2303a2a3061231e69d1a45ad432c061da3545
Clean up LanguageAndInputSettingsActivity since we already migrated to
LanguageSettingsActivity and InputSettingsActivity.
Bug: 330517633
Test: manual
Flag: EXEMPT bugfix
Change-Id: I32ef1bf4105355f1d11b728f51dcb3f7dc896c67