Expose "default tts locale" to the TTS V2 API.

This change allows TTS V2 API client applications to honor the
"default TTS locale" set by the user in the TTS settings.

Note that the TTS V1 API uses 3 character country/language codes
for locale encoding. It's the only system component that does
that. TTS V2 uses the standard, valid Locale objects. The default
TTS locale setting was stored as a 3 character locale string with
"-" as the separator.

This change switches the TTS locale setting format to the output of
Locale.toString() call (on a valid Locale object). Methods used for
reading this value can interpret both forms and try to return a valid
Locale object as an output.

Change-Id: Ice2e6c25a43eb9dd6e17d371ee582c2dae3329c9