fix: explicitly convert iOS volume to double

[chriszs]

What does this Pull Request do?

Fixes a suspected type conversion issue which made setVolume fail in some cases by explicitly converting volume to double from NSNumber (the type it's annotated in the React Native module) in Swift.

Why is this Pull Request needed?

To prevent setVolume from failing.

Are there any points in the code the reviewer needs to double check?

Someone who's more experienced with RN native development should double check me on this approach.

Are there any Pull Requests open in other repos which need to be merged with this?

No

Addresses Issue(s):

GitHub Issue

[Jmilham21]

Makes sense. I suppose we should also update setSpeed to avoid using Double as well.

@AmitaiB @david-almaguer please confirm but LGTM

[david-almaguer]

I'm fine using NSNumber instead of Double since this also matches the following method signature: RCT_EXTERN_METHOD(setVolume: (nonnull NSNumber *)reactTag :(nonnull NSNumber *)volume)

[chriszs]

@david-almaguer I suspect that signature may be why the type conversion issue happened in the first place, though I don't know the root cause for sure. double -> number is also supported by React Native.

[AmitaiB]

LGTM