Sample Build: emitGradle task depends on preflight

Add dependency to emitGradle task on preflight task to ensure that all
required sample files are present if emitGradle is run on a clean
checkout.

Change-Id: I860a6fdff4a5f9a9dcde6580df020f45786c1b0c
diff --git a/build.gradle b/build.gradle
index db5ca15..cea4d18 100644
--- a/build.gradle
+++ b/build.gradle
@@ -118,6 +118,7 @@
 }
 
 task emitGradle(type:Copy) {
+    dependsOn(preflight)
     def outputPath = outPath("gradle");
     def inputPath = "${project.projectDir}"
     // Copy entire sample into output -- since it's already in Gradle format, we'll explicitly exclude content that