Read endorsement VNB/VNA at the start of the signed structure

[erkki]

The UA-Signed Evidence Structure (RFC 9575 §4.1) is
[VNB (4)][VNA (4)][Evidence][UA DET (16)][UA Signature (64)].

handleEndorsementStructure advanced the buffer position past the signed content
before calling getInt(), so VNB/VNA were decoded from the first 8 signature bytes:
verifyTimestamps evaluated random values and rejected nearly every Wrapper/Manifest;
when the garbage happened to pass, evidenceSize went negative and Buffer.limit()
threw IllegalArgumentException on the scanner thread. Additionally both
handleWrapper/handleManifest received the whole signed structure instead of the
Evidence field: the wrapper % 25 check ran over VNB/VNA+DET, and the manifest
consumed VNB/VNA as the previous/current manifest hashes.

This is a regression from 1e08f74 ("DET verification"): the initial-commit
version read VNB/VNA in place and passed the Evidence slice to the handlers; that
refactor added the premature position() advance (invalidating the
evidenceSize computation written for the old cursor position) and swapped the
handler arguments from evidence to entire_message.

Change: read VNB/VNA in place, derive evidenceSize from messageSize, reject
structures shorter than VNB+VNA+DET, pass the Evidence slice to the handlers.
Device-verified: manifests pass timestamp, DET, and Ed25519 signature checks.

Tests: EndorsementStructureTest builds Manifest SAMs signed with a deterministic
Ed25519 test key and feeds them through handleSAMMessage. Against the previous
parsing, the positive cases fail (VNB/VNA decoded from signature bytes reject the
structure) and the tampered-signature case reproduces the Buffer.limit(-24)
crash. Also covers expired VNA, tampered signature, and truncated-structure
rejection. Test dependencies: androidx.arch.core:core-testing (synchronous
LiveData) and org.mockito:mockito-core — android.util.Log is statically mocked
per test class instead of enabling the module-global
unitTests.returnDefaultValues, which would silently default every android.jar
stub. The mockito line is identical in #4 and merges cleanly.