SourceStampVerification Return more specific error code when verification fails

Summary:
Upstream from Meta
If the source stamp tag is not present, SignatureNotFoundException is thrown.
In such cases , it is better to return with
SourceStampVerificationResult.notPresent() instead of
SourceStampVerificationResult.notVerified()
Also modified Android platform core tests (APCT) to reflect this change

Test: Verified using Android platform core tests (APCT)
All other tests pass successfully

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Change-Id: Ifb4b05b6c4a18646cb72fbdc03c529e80f798b21
Signed-off-by: sourabh-nanoti <sourabhn@meta.com>