Add (hidden) TTSEngines#isLocaleSetToDefaultForEngine method.

TTSEngines#isLocaleSetToDefaultForEngine allows the Settings application
to distinguish if the TTS engine locale is blank (set to follow the device
locale) or if it's set to the same value as the device locale.

Change-Id: I462584547d69c21cde4ac317c8a3b3bbc1680cae