[Android] [v2] Add support for external audio effect

[Alexandre Janniaux]

Here is the patch with the following fix compared to previous version:
+ return AUDIOTRACK_SESSION_ID without testing sdk version, as it should be 0 when not available.
+ don't rely on MediaSession.setActive to be immediate
+ fix exception on devices with sdk < 21

I've tested it from Android 4.0.4 to Android 8.