rxlabz
Add getVolume and setVolume for Android and iOS
I took the fork started by klinki to implement setVolume on Android, brought it current and then added getVolume. Also implemented both of these for iOS.
Since iOS uses a double from 0.0 to 1.0 for volume and Android uses a varying range of integers, I tried to normalize to 0 to 100 for all platforms. However, I am not an Android developer and I had trouble getting the functions for reading min and max volume on Android to work. So I commented that code out. It is still useful as is. But I think it would be better if the normalization worked in case someone more familiar with building for Android can get those to work.
I went ahead and forked this project and it is available at https://github.com/emostar/audioplayer and published at https://pub.dartlang.org/packages/audioplayer2.
When I get a chance, I'd like to see about adding this PR to the forked project, but it may be faster for you to do it yourself as you are more familiar with the changes.
