Improve TTS initialization and queuing.

This is the patch to improve TTS loading and reading that was [submitted on the forums](https://groups.google.com/forum/#!msg/anki-android/vggjjptptG4/fn8LaDORbjYJ). I cleaned it up a bit from the original source.

Changes:
- Don't loop over all TTS locales to find if current card's language is available.
- Cut question reading when card is flipped. Start answer immediately.
- Waiting dialog while TTS initializes.