Get rid of BpfCoordinator#mPollingStarted

Refactor IpServer/BpfCoordinator to remove mPollingStarted, and just
maintain the invariant that "data usage polling and conntrack
monitoring are running iff at least one IpServer is serving".

Test: atest TetheringTests
Bug: 294025403
Change-Id: I55064ee78e1cda12ad76b7c31496d760dcffdd76
5 files changed