[ci] Increase heavy workload memory (flutter#7065)

Android builds are increasingly running into OOM errors; this increases
the machine configuration from 12GB to 16GB to hopefully avoid the
issue.

Author: stuartmorgan-g

.cirrus.yml

@@ -228,7 +228,7 @@ task:
     zone: us-central1-a
     namespace: default
     cpu: 4
-    memory: 12G
+    memory: 16G
   matrix:
     ### Android tasks ###
     - name: android-platform_tests