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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etc/taskcluster/decision-task.py

@@ -15,11 +15,13 @@ def main():
     if task_for == "github-push":
         linux_tidy_unit()
         #linux_wpt()
+        android_arm32()
 
     # https://tools.taskcluster.net/hooks/project-servo/daily
     elif task_for == "daily":
         daily_tasks_setup()
         with_rust_nightly()
+        android_arm32()
 
     else:
         raise ValueError("Unrecognized $TASK_FOR value: %r", task_for)
@@ -70,6 +72,28 @@ def with_rust_nightly():
     )
 
 
+def android_arm32():
+    return decision.find_or_create_task(
+        index_bucket="build.android_armv7_release",
+        index_key=os.environ["GIT_SHA"],  # Set in .taskcluster.yml
+        index_expiry=build_artifacts_expiry,
+
+        task_name="Android ARMv7: build",
+        # file: NDK parses $(file $SHELL) to tell x86_64 from x86
+        # wget: servo-media-gstreamer’s build script
+        script="""
+            apt-get install -y --no-install-recommends openjdk-8-jdk-headless file wget
+            ./etc/ci/bootstrap-android-and-accept-licences.sh
+            ./mach build --android --release
+        """,
+        artifacts=[
+            "/repo/target/armv7-linux-androideabi/release/servoapp.apk",
+            "/repo/target/armv7-linux-androideabi/release/servoview.aar",
+        ],
+        **build_kwargs
+    )
+
+
 def linux_wpt():
     release_build_task = linux_release_build()
     total_chunks = 2