Prefetch audio instructions

[ericrwolfe]

Android version of mapbox/mapbox-navigation-ios#724

After each maneuver, we should prefetch the next set of instructions (and potentially more if a set of instructions are close together) to prevent latency from Polly requests or falling back to the default voice if out of cell service.

cc @cammace @danesfeder @bsudekum

[Guardiola31337]

I guess OP will be addressed after #751 lands. Could you confirm @devotaaabel?

[Guardiola31337]

#751 has landed 🎉

Are we good to close here @devotaaabel?

[danesfeder]

Going to close this unless @devotaaabel has any objections