DnsResolver: Make DnsException ctor public

Making the DnsException constructor public is useful for apps
using DnsResolver to write their tests, and for internal packages
that may want to implement the DnsResolver.Callback interface.

Test: for regression; CTS tests in b/208479811
Bug: 208464882
Change-Id: I14641688f53721c96e6df9596a7506912ba3aec0
2 files changed