Fix VPN settings UI to validate IPsec Identifier

This change ensures that the updateUiControls() (and validate()
transitively) is called whenever the IPsec identifier is updated.

Bug: 152811464
Test: Manual testing with settings UI
Change-Id: I6536652a60244c3fde3f65211f9d0709145c6cd9

src/com/android/settings/vpn2/ConfigDialog.java

@@ -173,6 +173,7 @@ class ConfigDialog extends AlertDialog implements TextWatcher,
         mProxySettings.setOnItemSelectedListener(this);
         mProxyHost.addTextChangedListener(this);
         mProxyPort.addTextChangedListener(this);
+        mIpsecIdentifier.addTextChangedListener(this);
         mIpsecSecret.addTextChangedListener(this);
         mIpsecUserCert.setOnItemSelectedListener(this);
         mShowOptions.setOnClickListener(this);