Simplest attempt at a fix for clear call log bug.

- Clearing the call log currently deletes voicemails from the log
  without removing their attachments (and leaving you no way of later
  deleting those attachments).
- The fix is either to not delete the voicemails when clearing the call
  log, or to delete their files as well.
- I think we should skip deleting voicemails, because having all your
  voicemails deleted is probably more than should be expected from a
  menu item entitled "clear call log".

Bug: 5123101
Change-Id: I665cee1ae94dc9c40826185ae9d01705d407022f