Support Android projects

[alexander-yevsyukov]

Currently ProtoData fails if it's added to an Android project. The stacktrace is this:

...
Caused by: java.lang.IllegalStateException: sourceSet should not be used in an Android project
	at com.google.common.base.Preconditions.checkState(Preconditions.java:502)
	at com.google.protobuf.gradle.GenerateProtoTask.getSourceSet(GenerateProtoTask.groovy:301)
	at com.google.protobuf.gradle.GenerateProtoTask_Decorated.getSourceSet(Unknown Source)
	at io.spine.protodata.gradle.plugin.PluginKt$configureProtobufPlugin$1$2.invoke(Plugin.kt:206)
	at io.spine.protodata.gradle.plugin.PluginKt$configureProtobufPlugin$1$2.invoke(Plugin.kt:201)
	at org.gradle.kotlin.dsl.KotlinClosure1.doCall(GroovyInteroperability.kt:94)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at org.gradle.util.internal.ClosureBackedAction.execute(ClosureBackedAction.java:73)
	at org.gradle.util.ConfigureUtil.configureTarget(ConfigureUtil.java:203)
	at org.gradle.util.ConfigureUtil.configure(ConfigureUtil.java:157)

This happens because of this code in the Gradle plugin (com.google.protobuf.gradle.GenerateProtoTask.groovy):

  @Internal("Inputs tracked in getSourceFiles()")
  SourceSet getSourceSet() {
    Preconditions.checkState(!isAndroidProject.get(),
        'sourceSet should not be used in an Android project')
    Preconditions.checkNotNull(sourceSet, 'sourceSet is not set')
    return sourceSet
  }

The first problem is that we use the internal API, which does not have a stability guarantee.
The second problem is we don't run under an Android project. I've stumbled on this issue when tried to create a test for checking that we don't add kotlin built-in for protoc if there's no Java or Kotlin enabled for a project.

The test is PluginTest.add 'kotlin' built-in only' if 'java' plugin or Kotlin compile tasks are present() and is currently disabled with the reference to this issue.