Trigger samples build manually via Gradle build property

Author: mttkay

rxjava-contrib/rxjava-android-samples-build-wrapper/build.gradle

@@ -1,13 +1,11 @@
-def androidHome = System.getenv("ANDROID_HOME")
 tasks.build.doLast {
-  if (androidHome.isEmpty()) {
-    println("No Android SDK detected; skipping Android samples build")
-  } else {
+  def androidHome = System.getenv("ANDROID_HOME")
+  if (project.hasProperty('buildAndroidSamples') && !androidHome.isEmpty()) {
     println("Android SDK detected at $androidHome, running samples build")
     project.exec {
       workingDir '../rxjava-android-samples'
 
       commandLine "./gradlew", "clean", "packageDebug"
     }
   }
-}
+}