AssetRedirectionService: Check for permissions before running dump()

Fix android.security.cts.ServicePermissionsTest failure

Change-Id: I9d629d19f3ff231db477d156c8bf557d4b8ee861
Don't assume any ringtone is present on external storage.

If that assumption doesn't hold true, the user will lose his ringtone
and won't know why.

Change-Id: I89409e1351792d020c5458f5104f4bb9f2a040f2