Fix the test flake on ConnectivityServiceTest

This test need a connected enterprise Network before setting
a profile network preferences. If there is no enterprise network
after setting no fallback preference, it will get lost callback
from cell network because it's not matched current preference.
The test fails sometimes because the enterprise network agent is
not fully connected yet. Thus, make sure the agent is fully
connected before settting a profile network preferences.

Fix: 242833379
Test: atest FrameworksNetTests:com.android.server.Connectivity\
      ServiceTest#testPreferenceForUserNetworkUpDownWithNo\
      FallbackWithAlreadyConnectedWorkAgent \
      --rerun-until-failure 100 on cf target
Change-Id: I040b9f31c29834f90f4ca62c512e392141a3244d