Change inlined okhttp3.Headers references to a Java import

Summary:
In BundleDownloader.java, there were references to `okhttp3.Headers` instead of importing the class at the top of the file. This PR is a simple change to use an import instead.

Compile the Android app

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/react-native-website, and link to your PR here.)

[ANDROID][MINOR][BundleDownloader] - Use Java import instead of fully qualified class name
Closes #17507

Differential Revision: D6691593

Pulled By: hramos

fbshipit-source-id: 81b366db608b7be8a903d2f25b36ca5642d9eec3

ReactAndroid/src/main/java/com/facebook/react/devsupport/BundleDownloader.java

@@ -28,6 +28,7 @@
 import javax.annotation.Nullable;
 import okhttp3.Call;
 import okhttp3.Callback;
+import okhttp3.Headers;
 import okhttp3.OkHttpClient;
 import okhttp3.Request;
 import okhttp3.Response;
@@ -171,7 +172,7 @@ public void execute(Map<String, String> headers, Buffer body, boolean finished)
                 if (headers.containsKey("X-Http-Status")) {
                   status = Integer.parseInt(headers.get("X-Http-Status"));
                 }
-                processBundleResult(url, status, okhttp3.Headers.of(headers), body, outputFile, bundleInfo, callback);
+                processBundleResult(url, status, Headers.of(headers), body, outputFile, bundleInfo, callback);
               } else {
                 if (!headers.containsKey("Content-Type") || !headers.get("Content-Type").equals("application/json")) {
                   return;
@@ -221,7 +222,7 @@ public void cancelDownloadBundleFromURL() {
   private void processBundleResult(
       String url,
       int statusCode,
-      okhttp3.Headers headers,
+      Headers headers,
       BufferedSource body,
       File outputFile,
       BundleInfo bundleInfo,
@@ -380,7 +381,7 @@ private static boolean isDeltaUrl(String bundleUrl) {
     return bundleUrl.indexOf(".delta?") != -1;
   }
 
-  private static void populateBundleInfo(String url, okhttp3.Headers headers, BundleInfo bundleInfo) {
+  private static void populateBundleInfo(String url, Headers headers, BundleInfo bundleInfo) {
     bundleInfo.mUrl = url;
 
     String filesChangedCountStr = headers.get("X-Metro-Files-Changed-Count");