Minimize layers in android-build dockerfile

As per the docker best practices [1], you should try minizing
the number of RUN, COPY and ADD directives to reduce how many layers
you use in an image.

Also clean out APT cache to reduce the image size.

[1] https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#minimize-the-number-of-layers

Test: cloud-build-local -dryrun=false .
Change-Id: I343727b4b6e42c914c02a20d7d0a4febd1855c68