Fix unused / unpropragated statuses in midi sample

[jfgoog]

For example, /*media_status_t status =*/ AMidiDevice_release(sNativeReceiveDevice);

Also changes introduced in https://github.com/android/ndk-samples/pull/866

[DanAlbert]

I wonder if we should be tagging all those APIs with [[nodiscard]] as well. I'll look into doing that when I get to this.