Add support for gid derivation on private volumes

This sdcardfs feature was moved under a mount option
and is only needed on private volumes

Test: Private emulated volume should attempt to mount with
      derive_gid option.
Bug: 63245673
Change-Id: I40a8b15c298c815a4643007b9eca8269379fd2ac

EmulatedVolume.cpp

@@ -84,6 +84,7 @@ status_t EmulatedVolume::doMount() {
                 "-g", "1023", // AID_MEDIA_RW
                 "-m",
                 "-w",
+                "-G",
                 mRawPath.c_str(),
                 label.c_str(),
                 NULL)) {