Fix Android, DevServerHelper from calling okhttp3 response.body.string() twice

Summary:
Fixing from call response.body.string() twice in DevServerHelper.java.
https://github.com/square/okhttp/issues/1240#issuecomment-68142603

I'm getting error like this
```
05-01 21:16:47.080 22836-23064/com.my.company.bundle E/AndroidRuntime: FATAL EXCEPTION: OkHttp Dispatcher
                                                                               Process: com.my.company.bundle, PID: 22836
                                                                               java.lang.IllegalStateException: closed
                                                                                   at okio.RealBufferedSource.rangeEquals(RealBufferedSource.java:398)
                                                                                   at okio.RealBufferedSource.rangeEquals(RealBufferedSource.java:392)
                                                                                   at okhttp3.internal.c.a(Util.java:449)
                                                                                   at okhttp3.v.string(ResponseBody.java:174)
                                                                                   at com.facebook.react.devsupport.f$8.onResponse(DevServerHelper.java:487)
                                                                                   at com.newrelic.agent.android.instrumentation.okhttp3.CallbackExtension.onResponse(CallbackExtension.java:41)
                                                                                   at okhttp3.s$a.c(RealCall.java:153)
                                                                                   at okhttp3.internal.b.run(NamedRunnable.java:32)
                                                                                   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
                                                                                   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
                                                                                   at java.lang.Thread.run(Thread.java:764)
```

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- In my case, My device is using a Proxy, and tried to connect the packager. When it failed from connecting packager through websocket, It crash by this line of code.
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[ANDROID] [BUGFIX] [DevServerHelper] - fixing from calling body.string() twice.

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Closes https://github.com/facebook/react-native/pull/19088

Differential Revision: D7853822

Pulled By: mdvacca

fbshipit-source-id: c11a73ce2fa6d40b0a7bd8bc0fca7b07c6bc27ed

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

@@ -479,10 +479,11 @@ public class DevServerHelper {
               callback.onPackagerStatusFetched(false);
               return;
             }
-            if (!PACKAGER_OK_STATUS.equals(body.string())) {
+            String bodyString = body.string(); // cannot call body.string() twice, stored it into variable. https://github.com/square/okhttp/issues/1240#issuecomment-68142603
+            if (!PACKAGER_OK_STATUS.equals(bodyString)) {
               FLog.e(
                   ReactConstants.TAG,
-                  "Got unexpected response from packager when requesting status: " + body.string());
+                  "Got unexpected response from packager when requesting status: " + bodyString);
               callback.onPackagerStatusFetched(false);
               return;
             }