Increase the maximum exponential backoff for DNS64 queries.

Currently, we only scale down DNS64 detection queries to one
every two minutes. This wastes power on true IPv6-only networks
such as the IMS networks of many operators. The only possible
benefit is that if a NAT64 were to appear on a previously
IPv6-only network, we detect it within two minutes.

Set the maximum backoff to one query per 30 minutes, while
maintaining the exponential backoff starting at 1 second.

Bug: 62650901
Test: bullhead successfully detects NAT64 network
Change-Id: Ia411018b92722615c3ac52bc7f360094bc96532f