Update debug counter in `testReplyPing`

Change debug counter from DROPPED_IPV6_MULTICAST_PING to DROPPED_IPV6_NS_REPLIED_NON_DAD.
The DROPPED_IPV6_MULTICAST_PING counter is no longer used and scheduled for removal.

Test: atest CtsNetTestCases:android.net.cts.ApfIntegrationTest
Change-Id: I00eac8522b4e18b9bae8bfeab9d451314f88b5f9
1 file changed