Fix DnsTlsSocket to consume all pending data from ssl

When servers reply multiple DNS responses in one packet, DnsTlsSocket
handles only the first DNS response. The remaining DNS responses
are still in ssl buffer. This causes a bug that there will be at
least one DNS reponse in ssl buffer, which results in at least one
DNS request timeout.

This change fixes it by always consuming the data from ssl before
next poll.

Bug: 172778187
Bug: 168027339
Bug: 171413368
Test: cd packages/modules/DnsResolver && atest
Change-Id: I72807e43636a46d30df6a694bb906313a8de63f2