Add L2TP secret, L2TP/IPSec PSK support. Fix screen orientation.

* Changes
  + Add L2tpActor, L2tpEditor, L2tpIpsecPskActor.
  + Make L2tpIpsecEditor extend L2tpEditor.
  + Revise the code for saving username. Make
    VpnSettings.saveProfileToStorage() static.
  + Fix support for screen orientation change in both VpnSettings and
    VpnEditor.

  Patch Set 2:
  + Remove Util.isNullOrEmpty(). Use TextUtils.isEmpty() instead.
  + Remove unused imports. Wrap lines longer than 80 chars.

  Patch Set 3:
  + Fix all the strings according to UI feedback.
  + Remove all the added actor subclasses and move password to editor.
  + Remove VPN entry in Security & location.

  Patch Set 4:
  + Misc string fixes.

  Patch Set 5:
  + Add strings for credential storage settings.
  + Changed the error dialog icon.
  + Fix "Remember me" indentation in connect dialog.

  Patch Set 6:
  + resolve res/values/strings.xml
15 files changed