Keystore 2.0: Make VPN Keystore 2.0 ready.

* Use public stable API to load certificates from keystore.
* Also use grants to allow racoon to use keystore keys without
  special exceptions in keystore.
* Use LegacyProfileStore instead of Keystore for storing VPN profiles.

Bug: 175068876
Bug: 171305607
Test: atest android.net.cts.Ikev2VpnTest
      atest android.net.cts.IpSecManagerTest
      atest com.android.server.connectivity.VpnTest
      atest com.android.server.ConnectivityServiceTest
Merged-In: I27975113896ea137260a9f94a34fb1c3ca173fe3
Change-Id: I27975113896ea137260a9f94a34fb1c3ca173fe3