Build Android engine in presubmit using Cirrus and GCE (#5882)

The whole process is less than 5 minutes using a 24-CPU GCE instance.

I may further improve it by using containers and more CPUs in the following PRs.

.cirrus.yml

+gcp_credentials: ENCRYPTED[987a78af29b91ce8489594c9ab3fec21845bbe5ba68294b8f6def3cf0d380830f06687a89ea69c87344c5ade369700fe]
+
+gce_instance:
+  image_project: flutter-cirrus
+  image_name: flutter-engine-ubuntu1604
+  zone: us-central1-a
+  cpu: 24
+  memory: 32Gb
+  disk: 60
+
+build_android_task:
+  env:
+    CIRRUS_WORKING_DIR: "/tmp/github_repo"
+    ENGINE_PATH: "/var/tmp/flutter/engine"
+    PATH: $PATH:/var/tmp/depot_tools
+
+  replace_engine_script: |
+    cd $ENGINE_PATH/src
+    rm -r flutter
+    cp $CIRRUS_WORKING_DIR -r ./flutter
+    gclient sync
+
+  compile_android_script: |
+    cd $ENGINE_PATH/src
+    ./flutter/tools/gn --android --unoptimized
+    ninja -C out/android_debug_unopt
+

travis/licenses_golden/licenses_flutter

@@ -187,6 +187,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 LIBRARY: engine
 ORIGIN: ../../../LICENSE
 TYPE: LicenseType.bsd
+FILE: ../../../flutter/.cirrus.yml
 FILE: ../../../flutter/DEPS
 FILE: ../../../flutter/lib/io/dart_io.cc
 FILE: ../../../flutter/lib/io/dart_io.h