Support for google-java-format's style option (AOSP style) - #193

The google-java-format CLI exposes an option --aosp to switch from the default Google Java Style to AOSP-compliant style (4-space indentation). Such a configuration option is missing from Spotless.

This PR adds a corresponding configuration option to Spotloss:

spotless {
  java {
    googleJavaFormat().aosp()
  }
}

Internally, google-java-format implements the style option as an enum class. Therefore, I've added a second, more general configuration option to switch to any possible enum value. However, as of version 1.5, the only values are GOOGLE (default) and AOSP.

spotless {
  java {
    googleJavaFormat('1.5').style('AOSP')
  }
}