Try to fix travis
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Revisit how the Android port works.
Fix#20855Fix#18625Fix#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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Use the new environment variable support for `buildbot_steps.yml`
to do this globally instead of having to patch every call,
which is error prone.
Set this variable globally in .travis.yml and appveyor.yml as well.
Mach: Add `mach clean-cargo-cache` command
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