Add "default_normal" support to vold.

This new flag isolates each user on a multi-user device for security
reasons.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.ExternalStorageHostTest#testSecondaryUsersInaccessible
Bug: 64672411
Change-Id: I3db8dde597a7715ca680779ac57957fb12a92f8e

model/EmulatedVolume.cpp

@@ -86,6 +86,7 @@ status_t EmulatedVolume::doMount() {
                 "-m",
                 "-w",
                 "-G",
+                "-i",
                 mRawPath.c_str(),
                 label.c_str(),
                 NULL)) {