Merge libnetddns into libnetd_resolv

libnetddns is the library for DNS-over-TLS and is statically
linked to netd. Deprecate it and move them to libnetd_resolv
as a more general DNS library for netd.

This change comprises:

[1] Clean up netd/server/dns/*. Move all DnsTls* files to
    netd/resolv/ to parts of libnetd_resolv library.

[2] Export DnsTls* classes being visible for netd. It will only
    be temporary for a while.

[3] Remove the libssl dependency in netd. The relevant stuff is
    moved to libnetd_resolv.

Note that DnsTls* classes are still required for DnsProxyListener
and ResolverController to manipulate private DNS servers even after
this change.

Bug: 113628807
Test: as follows
    - built, flashed, booted
    - system/netd/tests/runtests.sh
    - DNS-over-TLS in live network passed

Change-Id: Ieac5889b4ebe737f876b3dcbe1a8da2b2b1b629d
diff --git a/resolv/include/netd_resolv/IDnsTlsSocket.h b/resolv/include/netd_resolv/IDnsTlsSocket.h
new file mode 100644
index 0000000..4f21bbd
--- /dev/null
+++ b/resolv/include/netd_resolv/IDnsTlsSocket.h
@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef _DNS_IDNSTLSSOCKET_H
+#define _DNS_IDNSTLSSOCKET_H
+
+#include <cstddef>
+#include <cstdint>
+
+#include <netdutils/Slice.h>
+
+namespace android {
+namespace net {
+
+class IDnsTlsSocketObserver;
+class DnsTlsSessionCache;
+
+// A class for managing a TLS socket that sends and receives messages in
+// [length][value] format, with a 2-byte length (i.e. DNS-over-TCP format).
+// This interface is not aware of query-response pairing or anything else about DNS.
+class IDnsTlsSocket {
+public:
+    virtual ~IDnsTlsSocket() {};
+    // Send a query on the provided SSL socket.  |query| contains
+    // the body of a query, not including the ID bytes.  This function will typically return before
+    // the query is actually sent.  If this function fails, the observer will be
+    // notified that the socket is closed.
+    // Note that a true return value indicates successful sending, not receipt of a response.
+    virtual bool query(uint16_t id, const netdutils::Slice query) = 0;
+};
+
+}  // end of namespace net
+}  // end of namespace android
+
+#endif  // _DNS_IDNSTLSSOCKET_H