TestNetworkService: do not set IFF_MULTICAST when bringUp is false

Setting IFF_MULTICAST in TestNetworkService when the interface is being
brought up elsewhere creates a race condition that might clear the
IFF_UP flag (if TestNetworkService loses the race).

Test: atest EthernetManagerTest --iterations 10
Bug: 242343156
Change-Id: I86e1c8afdb82beed5202f027d906bcce8f963c82
2 files changed