Add support for starting IKEv2/IPsec VPNs from settings

This commit allows the startLegacyVpn() call to start Platform VPN
profiles, allowing Settings to use the IKEv2/IPsec VPN options

When using an aliased private key, the
Ikev2VpnProfile.KEYSTORE_ALIAS_PREFIX must be prepended to the front of
the alias. This will in turn result in the fromVpnProfile() function to
pull the key from the KeyStore, as opposed to the inline-key that the
VpnManager API uses.

Bug: 148991741
Test: FrameworksNetTests passing, new tests added in subsequent CL
Test: Manually tested
Change-Id: Icbca695c353b2e12e99305676404dbf1a4495949