buildserver: allow gradle/sdkmanager to install into the new m2repository

buildserver: allow gradle/sdkmanager to install into the new m2repository

Google is pushing gradle towards downloading all the SDK components that it
needs, rather than having a preconfigured SDK installed.  The buildserver
strongly supports the old model, with added checksum checking even. We can
still support the old model by pre-configuring the SDK and locking it down
as root. This can then also support the new model by setting the file perms
so that new packages can be auto-installed, but they cannot overwrite any
packages that come pre-installed and pre-verified.

fdroiddata!2096
closes #247