Bug 1396098 - part 3 - move android builds to new docker image; r=glandium

Now that we have a Docker image with newer library versions on it, we
can move our builds over.  The new images differ from the old
CentOS-based images in two important ways, though:

1) The system compilers in the new image are new enough to be used as
   host compilers; additionally, our CentOS-built GCC compilers will not
   work.  We need to change the Android mozconfigs to reflect that.  We
   also need to change the Android tasks to not depend on the GCC
   toolchain builds.

2) In a similar fashion, we can use the system JDK; we no longer need to
   use the JDK from the Android NDK, which we had packaged up via the
   Android dependencies task.

Both of these changes come with caveats: our l10n repack jobs continue
to run on the CentOS-based images; l10n repacks have not been completely
converted to Taskcluster.  So we need to:

1) Retain the use of our custom GCC toolchain for HOST_CC/HOST_CXX on
   the CentOS-based images.

2) Retain the JDK packages in the tooltool manifests, and referencing
   them when we build on the CentOS-based images.