Simplify enums in BandwidthController.

1. Ensure that the code always uses all enum values. This
   provides a clear compile-time error if a passed-in enum value
   is not handled, and allows us to remove several default
   case labels and unreachable error logging code.
2. Factor out to common functions the code that converts enum
   values to parts of iptables command lines.

Bug: 32073253
Test: netd_{unit,integration}_test pass
Change-Id: I7136055100dc312fa7cb8bba5506fe86412b1f4d