Refactor interface quota code before moving to iptables-restore.

This change substantially refactors the interface and shared
quota code in BandwidthController in preparation for making it
use IptablesRestoreController.

There should be no functional change.

Bug: 28362720
Test: bullhead builds,boots
Test: netd_{unit,integration}_test pass
Test: quota rules are added and removed when quotas are enabled/disabled
Change-Id: I7379fac58da4d98958bb050055a3c6bd7c617aa3