Auto merge of #20912 - paulrouget:androidv2, r=jdm,mortimergoro,simonsapin

Revisit how the Android port works.

Fix #20855
Fix #18625
Fix #21147

Before polishing and making sure everything works fine (like the VR code, the android-x86 version, the non-android version of the lib, …), I'd like to get some early feedback on the approach.

I recommend reviewing commit by commit.

To test, just follow the regular steps:

```
    ./mach build -d --android
    ./mach package --dev --android builds servo.apk
    ./mach install --dev --android && ./mach run --android
```

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