Add --android option to ./mach package

servo/servo#11210 recently updated `./mach
package` to make it a first-class Mach command and add support for
simple Linux packaging as well (in the form of tarballs). Android used
to be the only packaging target supported, so a naked `./mach package`
sufficed, but now we need to explicitly build the Android packaging.

buildbot/master/files/config/steps.yml

@@ -53,7 +53,7 @@ android:
 
 android-nightly:
   - ./mach build --android --release
-  - ./mach package -r
+  - ./mach package --android --release
   - s3cmd put target/arm-linux-androideabi/release/servo.apk s3://servo-rust/nightly/servo.apk
 
 arm32: