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

[ohorn]

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')
  }
}

[nedtwigg]

Fantastic, thanks very much @ohorn! Do you mind adding a bullet to https://github.com/diffplug/spotless/blob/master/plugin-gradle/CHANGES.md and https://github.com/diffplug/spotless/blob/master/CHANGES.md

I'll release this by Monday, if that's alright for your schedule.

[jbduncan]

I don't really have anything else to add here; this PR looks pretty durn good to me! Thank you very much from me as well @ohorn. :)

[ohorn]

Thank you. I've added some remarks to the CHANGES.md files. Please change if it doesn't fit.

[nedtwigg]

Published in libs 1.10.0 and plugin-gradle 3.10.0