Add Kotlin DSL extension function for configuring proto source directory set in Android builds

[voidzcy]

The plugin is missing the proto Kotlin DSL extension for Android builds.

protobuf-gradle-plugin/src/main/groovy/com/google/protobuf/gradle/ProtobufPlugin.groovy

Lines 209 to 215 in bf6301a

	getSourceSets().all { sourceSet ->
	String name = sourceSet.name
	SourceDirectorySet sds = project.objects.sourceDirectorySet(name, "${name} Proto source")
	sourceSet.extensions.add('proto', sds)
	sds.srcDir("src/${name}/proto")
	sds.include("**/*.proto")
	}

For Android builds, project.android.sourceSets returns AndroidSourceSets. So in Kotlin DSL it expects the receiver of proto extension function to be AndroidSourceSet, which is missing now.

Note the argument of the extension function should be an action for SourceDirectorySet instead of for AndroidSourceDirectorySet. This is because the plugin created SourceDirectorySet for both Android and non-Android projects.

Majority part of this change is for adding the integration test for Android builds with Kotlin DSL. The test project includes configuring custom proto source directories. Reviewer may only need to review build.gradle, src/main/kotlin/com/google/protobuf/gradle/ProtobufConfiguratorExts.kt, src/test/groovy/com/google/protobuf/gradle/ProtobufKotlinDslPluginTest.groovy and testProjectAndroidKotlinDsl/build.gradle.kts.

Resolves #429.

[ejona86]

@voidzcy, is this still supposed to be a draft?

[voidzcy]

@voidzcy, is this still supposed to be a draft?

Yeah, I haven't tested it yet. Also, this changes adds android build tool for compileOnly dependency. Do we want it?

[ejona86]

Also, this changes adds android build tool for compileOnly dependency.

I don't know Kotlin well enough to know the impact of that extension. I don't know what situations would trigger a NoSuchClassDefError or similar. If it is "safe" and "if that's the way you do it," then that can be fine.