ServiceConnection field removed from HttpTransport related classes

1. According to Javadoc and Droidoc 'each HttpURLConnection
instance is used to make a single request' only. Any attempt
to change HttpURLConnection properties or request data
after request is made will raise either IllegalStateException
or ProtocolException. So it is no sense to keep ServiceConnection
instance inside HttpTransport related classes.

2. Keeping ServiceConnection instance inside HttpTransport class
prevents this HttpTransport object from reusing.

It is good idea to remove ServiceConnection field from
HttpTransport related classes.