BpfCoordinator: avoid attach/deatach duplicate downstream program

The forwarding pairs may use the same downstream. Ex: 464xlat
tethering has rmnet0:ncm0, v4-rmnet0:ncm0 forwarding pairs.
Need to avoid duplicate downstream program.

Bug: 241106456
Test: atest BpfCoordinatorTest
Change-Id: Ic41b3ad856e4876808f8497a760081afcfd04988
2 files changed