Use doIpv6Provisioning in testPref64Option.

This is a simple refactoring to reduce code duplication, and it
shows that it's possible to use doIpv6Provisioning in tests that
also need to place their own expectations on LinkProperties
around provisioning time.

Bug: 152723363
Bug: 153694684
Test: atest NetworkStackNextIntegrationTests:IpClientIntegrationTest#testPref64Option --iterations 100
(clean cherry-pick from internal branch)
Merged-In: Iaa3e297e2417d4cdf9393baf27d61657e5743888

Change-Id: Iaa3e297e2417d4cdf9393baf27d61657e5743888