OpenSL ES is the library used for native audio processing on Android. OpenAL (the library, not the Dolphin backend) is not available for Android (the operating system, not the port of Dolphin). The OpenAL audio backend is not a part of Dolphin for Android for the same reason the Direct3D 11 graphics backend isn't included.
As for asynchronous audio, the many, many reasons why Dolphin intentionally does not allow this are thoroughly documented (Google if you are interested) and are still true on Android. It will not be added to the OpenSL ES backend.
As for asynchronous audio, the many, many reasons why Dolphin intentionally does not allow this are thoroughly documented (Google if you are interested) and are still true on Android. It will not be added to the OpenSL ES backend.