Removed usage of internal parts of the OkHttp3.

[tavar]

Replaced the calls of internals with direct interaction with the Android SDK without OkHttp in the middle.
Allow updating to newer versions of OkHttp.

Fixes #70

[coveralls]

Coverage remained the same at 25.49% when pulling 967c4c4 on tavar:removedInternalPartsOfOkHttp into 8db4f00 on ibm-bluemix-mobile-services:master.

[AnanthaKrish]

@tavar Merging the code to development branch . After testing will release a new version. Thank you.

[Sadhvi-Nayak]

@AnanthaKrish Is this in new version? facing this issue for push notification service.

java.lang.NoSuchFieldError: No static field UTF_8 of type Ljava/nio/charset/Charset; in class Lokhttp3/internal/Util; or its superclasses (declaration of 'okhttp3.internal.Util' appears in /data/app/<app>==/base.apk)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at com.ibm.mobilefirstplatform.clientsdk.android.core.internal.ResponseImpl.getResponseText(ResponseImpl.java:120)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at com.ibm.mobilefirstplatform.clientsdk.android.core.internal.ResponseImpl.toString(ResponseImpl.java:267)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at com.ibm.mobilefirstplatform.clientsdk.android.push.internal.MFPPushInvoker.onSuccess(MFPPushInvoker.java:97)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at com.ibm.mobilefirstplatform.clientsdk.android.core.api.Request$1.onResponse(Request.java:467)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at okhttp3.RealCall$AsyncCall.execute(RealCall.java:174)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
07-02 21:12:20.652 25962 26056 E AndroidRuntime: 	at java.lang.Thread.run(Thread.java:919)