Speed up the DHCP roaming tests.

The DHCP roaming tests all call doDhcpRoamingTest. This method
appears to want to have the IpMemoryStore return null, but
instead just causes DhcpClient's IpMemoryStore query to time out.
Fix this by actually calling the onNetworkAttributesRetrieved
callback with null.

Before:
real	0m43.728s

After:
real	0m38.660s

Bug: 152723363
Test: atest NetworkStackNextIntegrationTests
Change-Id: I6fc93f82fda6891b3973d04e30cdfce6563077a1