String fixes for the credential storage and vpn settings.

String fixes for the credential storage and vpn settings.

* Changes
  + Shorten the messages for both set-password and enter-password dialogs
    when other activities access the credential storage.
  Patch Set 2:
  + Fix the capital issue in the vpn error messages.
    + Add separate strings for terms used in the error messages.
    + Modify related src files that use those terms.