Prevent SD-card from Un-mounting if cancelled during erase

If User cancels during the formatting stage
of "Erase SD Card", make sure you do not
Un-Mount the Volume

CRs-Fixed: 616156

Conflicts:
        core/java/com/android/internal/os/storage/ExternalStorageFormatter.java

Change-Id: I8d7e1cf64917ee3628a1a682e2f3fb32443f3a2e