Add new Credential prefix for platform VPNs

Platform VPN profiles keyed by apps must use a different key prefix to
prevent the Settings app from seeing them (and thus exposing potentially
sensitive credential information). Thus, the profiles will be
partitioned as follows:

Custom (Settings-app) based profiles continues using VPN_* prefix
App provisioned profiles will use the new PLATFORM_VPN_* prefix

Bug: 144245359
Test: Compiles, FrameworksNetTests passing
Change-Id: If8762e468c6cbbeb80738f14b066ebbad5cc665f