BandwidthController - don't collect egress interface stats from clatd

While we're at it we also add some hopefully useful comments.

Test: atest
Bug: 150738490
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: If20ff6949fd86591d882b63d012978db53737c7c
Merged-In: If20ff6949fd86591d882b63d012978db53737c7c