Move android_ network functions to LIBC_PRIVATE

Make following functions platform-private:
 android_getaddrinfofornet
 android_getaddrinfofornetcontext
 android_gethostbyaddrfornet
 android_gethostbynamefornet

Bug: http://b/26794226
Change-Id: I0855b0b82721e4d24e918508931b4d5587fcae7d
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index e5933ae..b8e9f3e 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -171,10 +171,6 @@
     alarm;
     alphasort;
     alphasort64;
-    android_getaddrinfofornet;
-    android_getaddrinfofornetcontext;
-    android_gethostbyaddrfornet;
-    android_gethostbynamefornet;
     android_set_abort_message;
     arc4random;
     arc4random_buf;
@@ -1182,6 +1178,10 @@
 
 LIBC_PRIVATE {
   global:
+    android_getaddrinfofornet;
+    android_getaddrinfofornetcontext;
+    android_gethostbyaddrfornet;
+    android_gethostbynamefornet;
     free_malloc_leak_info;
     get_malloc_leak_info;
     gMallocLeakZygoteChild;