Fix bug 2577511 NPE in TTS Settings

A previous bug had caused incorrect settings to be saved, in
particular the default language variant was not stored, unlike
the default language and country. When settings were restored, this
caused an invalid group of settings to be loaded, with a null
language variant. The fix consists in:
- discarding invalid default language settings
- reevaluating which language should be selected in the list of
  available languages.

Change-Id: If58882a9acc28f14a78f6142fefa17d04ca4ad5d