Remove duplicated bpf offload support check in IpServer

In current code, IpServer will ensure bpf offload is supported before
calling BpfCoordinator's APIs. But those APIs already have the same
checks inside the function. This CL removed duplicated checks in
IpServer.

After this change, the BPF offload support status is only determined
inside BpfCoordinator and it won't change during the life cycle of
Tethering. Tethering initializes mBpfCoordinator just once in the
constructor. After that point, the value of isUsingBpf can never
change, because mIsBpfEnabled is final and
mBpfCoordinatorShim.isInitialized() is either always true (api30) or
can either never change (api31).

Also fixed a bug in IpServerTest upon this change.

Test: atest TetheringTests
Bug: 261923493
Change-Id: I50f231a83bf678b90eae4fd9bfa6035e86978bb3