ACTUALLY disable dokka builds (#5005)

Summary:
This should hopefully unblock our Android builds for real this time. I didn't realise that the AGP itself seems to ship with a version of Dokka, so just removing the import didn't actually address the problem we were seeing.

Pull Request resolved: https://github.com/facebook/flipper/pull/5005

Test Plan:
```
./gradlew publishToMavenLocal -PRELEASE_SIGNING_ENABLED=false
```

This now succeeds.

Reviewed By: jknoxville

Differential Revision: D48312784

Pulled By: passy

fbshipit-source-id: f049e089a511726857f544386fe13dcb50fb2971

build.gradle

@@ -41,6 +41,10 @@ subprojects {
     tasks.withType(Javadoc).all {
         enabled = false
     }
+
+    tasks.withType(com.android.build.gradle.tasks.JavaDocGenerationTask).all {
+        enabled = false
+    }
 }
 
 ext {