BpfCoordinator: refactor updateUpstreamNetworkState to get MTU

This is a preparation for CLAT offload.

Move the getting the MTU into a function for refactoring
updateUpstreamNetworkState() which could be simpler.

Bug: 258637850
Test: atest BpfCoordinatorTest
Change-Id: I8f22b022d4d00597810b56ab6e3e55ed7ab9076d