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.

src/com/android/settings/vpn/L2tpIpsecEditor.java

@@ -55,10 +55,10 @@ class L2tpIpsecEditor extends L2tpEditor {
     public String validate() {
         String result = super.validate();
         if (result == null) {
-            result = validate(mUserCertificate, R.string.vpn_user_certificate);
+            result = validate(mUserCertificate, R.string.vpn_a_user_certificate);
         }
         if (result == null) {
-            result = validate(mCaCertificate, R.string.vpn_ca_certificate);
+            result = validate(mCaCertificate, R.string.vpn_a_ca_certificate);
         }
         return result;
     }