Bug 1396098 - part 0b - rename android-gradle-build to android-build; r=glandium,dustin

The name `android-gradle-build` is an accident of history; let's rename it
before we attempt major surgery on it.

--HG--
rename : taskcluster/docker/android-gradle-build/Dockerfile => taskcluster/docker/android-build/Dockerfile
rename : taskcluster/docker/android-gradle-build/README.md => taskcluster/docker/android-build/README.md
rename : taskcluster/docker/android-gradle-build/REGISTRY => taskcluster/docker/android-build/REGISTRY
rename : taskcluster/docker/android-gradle-build/VERSION => taskcluster/docker/android-build/VERSION
rename : taskcluster/docker/android-gradle-build/buildprops.json => taskcluster/docker/android-build/buildprops.json
rename : taskcluster/docker/android-gradle-build/dot-config/pip/pip.conf => taskcluster/docker/android-build/dot-config/pip/pip.conf
rename : taskcluster/docker/android-gradle-build/oauth.txt => taskcluster/docker/android-build/oauth.txt