Cope with rkp-only devices in legacy keyblob test

On a rkp-only device it is not possible to generate an attestation
on a bare key generation (attestations can only be generated if an
ATTEST_KEY is provided).

Bug: 329409739
Test: keystore2_legacy_blobs_test
Change-Id: Icdc4037466ab32366c34feeb42b58822ea110ebf
1 file changed