Restore device locale from backup.

The locales are merged with following policy.
- Don't change UI locale.
- Don't add unsupported locales.
- Don't add duplicated locales.

Bug: 35391006
Test: com.android.providers.settings.SettingsHelperTest
Test: Did the following tests manually.
1. Login with Google account during SUW.
2. Set locale to "zh-TW,en-US"
3. adb shell bmgr backupnow com.android.providers.settings
4. fastboot flash userdata && fastboot reboot
5. adb reboot bootloader
6. fastboot flash userdata && fastboot reboot
7. Choose "Japanese" as the first menu on the SUW.
8. Backup from cloud with logging in to the Google account.
9. After compete SUW, verify the device locale is "ja-JP,zh-TW,en-US"

Change-Id: I1e6c7ba5b7abb6bde8b01ce0f647c04a5caa81a6