Make LocalePicker Fragment.

The logic calling selectFirst() is removed as
- There's no comment why it is needed.
- Actually SetupWizard gets stack as that forces users to see
  WirelessSettings in SetupWizard.

The other changes:
- Move back LocalePickerFragment to LocalePicker.
- Make <activity> for LocalePicker in AndroidManifest <activity-alias>
- Add a short comment about how getComponent(), which should be a key
  for understanding how top-level settings work.
- Modify LanguageSettings so that it corectly points LocalePicker as
  a fragment.

Change-Id: I78d356e40af896ba1aab72fba12c90467371c7b0