fix DSL in Android plugin

Fix extra "kotlin" node in DSL for Android-gradle plugin.
Before
```groovy
android {
 sourceSets {
        main {
            kotlin {
                kotlin {
                    srcDirs = ['root/kotlin']
                }
            }
        }
}
```
After
```groovy
android {
  sourceSets {
         main {
             kotlin {
                 srcDirs = ['root/kotlin']
             }
         }
     }
}
```