Don't strip country and variant from locale.

When the synthesizer returns LANG_AVAILABLE or LANG_COUNTRY_AVAILABLE
for a certain locale, we strip the country and variant from the locale
before asking the synthesizer for the default voice name for this
locale.

This can prevent the synthesizer from picking the right locale. For
instance it prevents the synthesizer from picking en-gb if en-au is
requested, now it can only have a single preferred voice for all English
countries.

The solution is not to strip the country and variant from the locale.
This is not likely to break anything, because the input to
onGetDefaultVoiceNameFor was never expected to be a subset of all
supported language-country-variant combinations. The default
implementation handles this correctly.

Change-Id: I017f69d321039c7aa7a3ef5a09ba138e35b66670
(cherry picked from commit 4758e99783493fc546e6819b5fe9d44e1ef8c318)