Fix: IKEv2 RSA VPN crash on null Keystore private key encoding

IkeV2VpnRunner crashed when setting up an IKEv2/IPSec RSA type VPN
connection. The crash occurred because the RsaPrivateKey encoding was
null when transferring Ikev2VpnProfile to VpnProfile, specifically
when handling KEYSTORE_ALIAS for the private key. This resulted in a
NullPointerException in encodeForIpsecSecret because the 'secret' was
missing. This change corrects the serialization logic in
Ikev2VpnProfile to properly handle and transfer Keystore-backed
private key aliases, preventing the secret from becoming null during
the profile conversion.

Bug: 429063885
Test: Manually verify that an IPSec RSA type VPN can be set up
      successfully.
Flag: EXEMPT bug fix
Change-Id: Ia7aaf017b28bdaa27639db826ab9dc30a46833db