Auto merge of #21835 - servo:tc-android-arm32, r=jdm
Auto merge of #21835 - servo:tc-android-arm32, r=jdm

Taskcluster: add Android build

[Example green build](https://tools.taskcluster.net/groups/FyF53cdfSKC39eqB3OAZVQ/tasks/ff23qoM3TzSCHrinmzU7gQ/details), with `servoapp.apk` in public artifacts.

This installs some dependencies within the task for now. I’ll follow up with moving some things like the NDK download to a Docker image, but even as it is the task completes in 17 minutes.

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