Rephrase dialog message of clear storage dialog for security concern am: 7fd0ece6 am: 204f82d6 am: ac38fc7f

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Settings/+/16101569

Change-Id: I866f203650beccf90f77ced375bf3b41adc358f1