core: Use pvmfw_embedded_key for INTERNAL_PVMFW_EMBEDDED_AVBKEY

Instead of wrongly using the key that was used to sign pvmfw, we should
have been using the key that pvmfw will use to verify payloads it
receives so fix that.

Bug: 255575669
Test: -
Change-Id: I68a87a092c1aa4a60f4fbe60c5ddf82c4e34c38a