Add connectivity flag

Currently the muzima-api relying on the HTTP timeout to decide whether the device have internet connection or not. When the socket timeout, it will gracefully handled but this potentially forcing user to wait.

A better approach would be to allow the device to tell muzima-api whether connectivity is available or not. This would be a setting for muzima-api to whether skip internet based operation (download or upload) or not.

Another approach would be to check the internet connection in the android application and skip the internet based operation when the device don't have connectivity.