Fix usage of BandwidthUsage.create()

The _poll_bandwidth_usage periodic task was attempting to use
BandwidthUsage as a classmethod even though it is not defined as a
classmethod.  It has been updated to be used properly and a test was
added to ensure there is no error.

Change-Id: I96709675a952857da5121da29acd1ab2e6726fd0
Closes-Bug: #1392423
Merge "Fix usage of BandwidthUsage.create()"