Keystore: suggest vendor properties for device IDs

Drop the suggestion around using ro.product.<device-id>_for_attestation
values when running GSI tests, and instead recommend setting the
ro.product.vendor.<device_id> properties (which should survive GSI
intact.

Bug: 308122994
Test: CtsKeystoreTestCases:android.keystore.cts.KeyAttestationTest
Change-Id: I6b796abc4174ca1e6158003395419708e0268afa
(cherry picked from commit 785c5dd2e5694fdc7be2a030f125aa700db1d823)
Merged-In: I6b796abc4174ca1e6158003395419708e0268afa