Add install-cert-from-sdcard in Security settings

Also:
* Resume keystore states in SecuritySettings.onResume().
* Use action strings defined in CertTool and Keystore.

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

@@ -652,7 +652,7 @@ public class VpnSettings extends PreferenceActivity implements
         if (isKeystoreUnlocked()) return true;
         mUnlockAction = action;
         startActivity(
-                new Intent(SecuritySettings.ACTION_UNLOCK_CREDENTIAL_STORAGE));
+                new Intent(Keystore.ACTION_UNLOCK_CREDENTIAL_STORAGE));
         return false;
     }