Support android plugin

[anserran]

This PR is related to #17
I tried your changes but they didn't work, so I started digging. It seems that the android plugin follows the same structure as the java plugin, but the sourceSets are implemented with different classes.
It seems necessary to manually walk through all the android directories to collect the files to be formatted.
This is my first time working with groovy and gradle plugins, so I could've easily done something awful in the code.

[youribonnaffe]

Indeed I did not check the API for the Android plugin and assumed it was similar to the Java plugin.
Did you get a specific error when testing?

Although different it seems the API is close, with a getJava() method that returns a SourceDirectorySet, perhaps it is the plus() method that is not implemented?

[anserran]

Yes, the exact Exception is:

No signature of method: com.android.build.gradle.internal.api.DefaultAndroidSourceDirectorySet.plus() is applicable for argument types: (com.android.build.gradle.internal.api.DefaultAndroidSourceDirectorySet) values: [[src/test/java]]
  Possible solutions: is(java.lang.Object), split(groovy.lang.Closure), use([Ljava.lang.Object;), wait(), grep(), dump()

(This is after changing file?.isEmpty() for !file. Otherwise, condition was always false)

[youribonnaffe]

Perhaps it is possible to create a file collection from those without the plus method.

[anserran]

Yes, that's what I tried in this PR. DefaultAndroidSourceDirectorySet doesn't have a method that returns all the files, so I added a method to FormatTask.groovy.