Fix TestNetworkAgentWrapper flakiness

The underlying issue here is that before aosp/986824 the call to
waitForIdle is the local ConnectivityServiceTest function, but
after that patch TestNetworkAgentWrapper has an identically
named member that only wait for its own handler.

Bug: 141341704
Test: ConnectivityServiceTest --generate-new-metrics 100
Change-Id: I38709cc69ec7a261c1c6e008abe03d2882e2083d