Use a Debian9 container because the build scripts for Flutter Android require jdk8, which isn't available on Debian10.
-Shell script executes the GN and Ninja commands
-flutter.py executes the script in Debian9 container
--the Flutter build is particular about directory structure and uses various relative paths, so the entire swarming directory is mounted.
-Followup CL will update remove the old Flutter Android job and update the BuildStats job.
Bug: skia:9438
Change-Id: I60c62425f21214b4ae6a810a787418a8975d204e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282616
Commit-Queue: Weston Tracey <westont@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
