Switch to a new way of activating DNS-over-TLS

This change removes the global database of potential DNS-over-TLS
servers from Netd, and makes pinned or named servers mandatory-TLS,
not opportunistic.

Bug: 64753847
Change-Id: I226ffec3f59593bc40cd9019095c5261aae55fa0
Test: Tests pass.  Normal browsing continues to work normally.
12 files changed