netd: BandwidthController: Allow tracking loopback interfaces

In the past, loopback interfaces were of no use for bandwidth/datausage.
We now don't treat loopback interfaces any differently.
This will helps with CTS tests that checking the kernel's qtaguid functionality
and the matching user-space handling.

Change-Id: If307ceebc10f89269d1b051f666ec666e5e38d41