Rephrase dialog message of clear storage dialog for security concern am: 7fd0ece6f4 am: 204f82d6c4

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

Change-Id: Ifdb5eb3c216bab863cdf3e641bf1a050c0e5ee74