Fix flaky testNattSocketKeepalives

The test did not wait for the WifiNetworkAgent to disconnect, and there
are reports of flakes where it is still connected in
connectKeepaliveNetwork.

Test: atest FrameworksNetTests
Bug: 124319553
Change-Id: I85968806885c8853ec4f4dbea356366c1af0ea1c
1 file changed