Add imports to networking test mapping

Ensure that appropriate networking tests are triggered when submitting
changes in the frameworks/base directories.

Change-Id: Ibf8b567fa7721299120b0a6c6836dd770d92a9fc
Test: treehugger runs TEST_MAPPING tests

services/net/TEST_MAPPING

@@ -0,0 +1,7 @@
+{
+  "imports": [
+    {
+      "path": "frameworks/base/core/java/android/net"
+    }
+  ]
+}