[BOT.6] Make BpfCoordinator to support data limit

The BPF tethering coordinator listens to the forwarding rule
changes for updating data limit which is applied in the following
conditions.
- After adding the first rule on a given upstream, add data limit.
- After removing the last rule on a given upstream, clear data limit.
- The service applies a new data limit on current upstream.

The reason for relying on rule changes is because the Tethering and
IpServer objects have multi-internal state machines. It is hard to
synchronize all of their states.

Note that the data limit cleanup for stopping or switching upstream
relies on offload rules are all removed as well.

Bug: 150736748
Test: manual
Original-Change: https://android-review.googlesource.com/1302436
Merged-In: I829d36339973f9473fe6b616c48aa288f18d1c46
Change-Id: I829d36339973f9473fe6b616c48aa288f18d1c46