Move jni file descriptor helper code to libnetjniutils

Enables sharing of code with other modules.

Bug: 158749603
Test: m
Change-Id: I6d1e3d41047505d1e803fd46cfef22f14b731b55
diff --git a/Android.bp b/Android.bp
index e09eaab..d6519b2 100644
--- a/Android.bp
+++ b/Android.bp
@@ -237,6 +237,9 @@
         "liblog",
         "libnativehelper_compat_libc++",
     ],
+    static_libs: [
+        "libnetjniutils",
+    ],
 
     // We cannot use plain "libc++" here to link libc++ dynamically because it results in:
     //   java.lang.UnsatisfiedLinkError: dlopen failed: library "libc++_shared.so" not found