Set default language in new TTS clients.

A recent change altered semantics of getLanguage call to return client
language instead of service language. This solved problems
with interferences between two clients using different lanaguages.

This change created a bug - new TTS client instance have no language set.
Since reading user preferences requires additional permissions I've
added new tts service method - getClientDefaultLanguage that will return
user preferences.

I've also added new client method, getDefaultLanguage, that allow easy
access to this data.

Bug: 7666482
Change-Id: Ieb7d2ba3a99d20c513add97f054874720a1cd82e