use system-installed Android utilities
This series of commits adds the ability for fdroid to use system-installed utilities like aapt, adb, and zipalign. Now that those three are packaged and included in Debian, fdroid should be able to use them. Once this merge request is accepted, then I'm going to include the directly relevant ones into the fdroidserver Debian package, and try to get that included in the jessie release (wish me luck getting that by the release masters).
This merge request should not change the functionality at all when sdk_path and build_tools are set in config.py.