Implement INetd.aidl getProcSysNet().

Also: a few "tidy-inspired" changes.

Test: as follows
    - built, flashed, booted
    - system/netd/tests/runtest.sh passes
Bug: 32163131
Change-Id: Icaa164af3c3d0d03af1ec083dfcbe856ac51529f
diff --git a/server/InterfaceController.h b/server/InterfaceController.h
index 58505a6..45ff878 100644
--- a/server/InterfaceController.h
+++ b/server/InterfaceController.h
@@ -47,6 +47,9 @@
 
     // Read and write values in files of the form:
     //     /proc/sys/net/<family>/<which>/<interface>/<parameter>
+    //
+    // NOTE: getParameter() trims whitespace so the caller does not need extra
+    // code to crop trailing newlines, for example.
     static int getParameter(
             const char *family, const char *which, const char *interface, const char *parameter,
             std::string *value);