[TIMOB-24983] Android: Add "HTTPClient.responseHeaders" support (feature parity with iOS)

*Summary:* This is a feature parity issue between Android, iOS, and Windows Phone when using HTTPClient. On iOS, you access the HTTP response's headers via the "responseHeaders" property, which returns them as a dictionary... http://docs.appcelerator.com/platform/latest/#!/api/Titanium.Network.HTTPClient-property-responseHeaders On Android and Windows Phone, you have to access the headers via the "allResponseHeaders" property instead, which returns all headers concatenated to a single string... http://docs.appcelerator.com/platform/latest/#!/api/Titanium.Network.HTTPClient-property-allResponseHeaders *Solution:* Add "responseHeaders" property support to Android. The way iOS is doing it (returning a dictionary) is the superior solution. (In the future, we should add support to Windows as well.) *Note:* Check if iOS includes the response's "status line" in 1 of it's returned headers. This is because Android's Java HttpURLConnection.getHeaderFields() method does return the status line in the returned HashMap, which is using a null key. In my opinion, we should not return status line as a header in JavaScript (this is what our HTTPClient.statusText property is for), but we should double check our iOS implementation and follow what it does.