TtsService.java: use the context from the TtsService object itself
TtsService.java: ensure that the voice data is installed/upgraded correctly.

When using assistive technology like TalkBack and not accessing
either the eSpeak application or the Android TTS settings
directly, the voice data would previously not be installed/upgraded
correctly.

The approach here is to move the logic to onIsLanguageAvailable so
that if the espeak voice data is not installed or is upgradable
then trigger an install of the voice data and return
TextToSpeech.LANG_MISSING_DATA.