Auto merge of #21158 - ferjm:webaudio, r=manishearth,nox,ferjm

WebAudio API

- [X] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #6710
- [X] There are tests for these changes

This PR adds basic support for the WebAudio API using [servo-media](https://github.com/servo/media) with GStreamer as the audio backend.

There are still some major stuff to fix like:

- [x] Detach ArrayBuffer during the [AudioBuffer "acquire the content" operation](https://webaudio.github.io/web-audio-api/#acquire-the-content). I am naively using `JS_StealArrayBufferContents()` directly, because it is what Gecko uses, but this should probably be part of the [rust-mozjs](https://github.com/servo/rust-mozjs) [TypedArray](https://github.com/servo/rust-mozjs/blob/master/src/typedarray.rs) API. And, in any case, I am not even sure if that's the proper way to do it. According to the results of the WPTs it may not even be right since [this assertion](https://github.com/servo/rust-mozjs/blob/master/src/typedarray.rs#L285) is failing in some cases. I need to dig more about this.
- [x] Disable the GStreamer dependency on Android. Unfortunately gstreamer-rs requires an NDK version upgrade, so we need to disable this for Android until then. I tried adding [different features to servo-media](https://github.com/servo/media/pull/79), but I am currently hitting [this issue](https://github.com/rust-lang/cargo/issues/1197)

I still need to run servo-tidy, change the servo-media dependency to use the git repo and add/fix some comments and TODOs.

The remaining feature work should be done in future PRs.

Note that most of the failing WPTs are failing because we don't implement the tested features yet (we only implement a few AudioNodes) and we have no OfflineAudioContext support, which most WPTs rely on.

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