Fix testIpClientServiceAvailableAfterPolling

The test used NetworkStack.setServiceForTest(null) expecting that null
would be returned in NetworkStack.getService, which is incorrect: using
null just causes getService to use default system behavior, which
returns the actual service.

Bug: 139314310
Test: atest ModuleNetworkStackClientTest --rerun-until-failure 500
Change-Id: I2d607d3ab26bcecd20546041ac4be2892a3fb852