multi-instance support

[TinfoilSubmarine]

Not sure if this is desirable since it deviates from the upstream mpv-android, but figured I would put it here since I needed it and thought others might find it useful.

EDIT: should probably document what this actually does. You can now do this, whereas before you would be limited to a sort of "singleton" mode.

val mpvOne: MPVLib = MPVLib()
mpvOne.create(context)
mpvOne.setOptionString("vo", "gpu")
mpvOne.setOptionString("gpu-context", "android")
mpvOne.setOptionString("hwdec", "auto")

val mpvTwo: MPVLib = MPVLib()
mpvTwo.create(context)
mpvTwo.setOptionString("vo", "gpu")
mpvTwo.setOptionString("gpu-context", "android")
mpvTwo.setOptionString("hwdec", "no")