Move resolver public headers to separate include path

Additionally rename functions and structs to eliminate those ugly
underscores from the public API (with the notable exception of
__res_params, which will come in its own separate change to keep this
one from growing too large).

Test: build, flash, then 'atest netd_integration_test'
Change-Id: I7e9b0ee6cdbec889e9c35b5a17b5daa65533686e
diff --git a/server/NetdNativeService.cpp b/server/NetdNativeService.cpp
index 3e76fcf..cafb90e 100644
--- a/server/NetdNativeService.cpp
+++ b/server/NetdNativeService.cpp
@@ -25,7 +25,6 @@
 #include <android-base/strings.h>
 #include <cutils/properties.h>
 #include <log/log.h>
-#include <resolv_netid.h>
 #include <utils/Errors.h>
 #include <utils/String16.h>
 
@@ -46,6 +45,7 @@
 #include "RouteController.h"
 #include "SockDiag.h"
 #include "UidRanges.h"
+#include "netid_client.h"  // NETID_UNSET
 
 using android::base::StringPrintf;
 using android::net::TetherStatsParcel;