Accept delegated prefix with length shorter than 64.

The prefix length shorter than 64 is still available for slaac, we
should accept it and construct the IPv6 address based on that prefix.
Also fix the prefix length used in the integration test which is always
64, that's incorrect, it results in the test case intended to issue a
shorter prefix length can still pass.

Bug: 320683437
Test: atest testDhcp6Pd_shortPrefixLength --iterations
Change-Id: I6bcc5534c4a116e84d07e941afd451dd148d15cd