android-sdk: Update Wrapper Scripts

- Clean up some of the user facing copy.
- Add examples of how to install the various tools required to use the sdk.
- Change the interpreter of wrapper scripts to /usr/bin/env bash.
- Clean up the wrapper scripts.
- Set a variable for the version of the Android Build Tools we want to use.
- Change the default version of the Android Build Tools to 19.0.3.
- Add an environmental variable check in the build-tools wrappers to
  let users change the version of the build-tools
  they want to execute by default.