Catch all exceptions to verifyAttestion() in AVF

Re-structure the code in AttestationVerificationPeerDeviceVerifier in a
way so that all exceptions could be caught in verifyAttestation() (the
only "public" method of the class) and thus prevent exception to
escaping out of the class.

Fix and slightly expand javadocs in
AttestationVerificationPeerDeviceVerifier.

Test: atest AttestationVerificationTest
Change-Id: Ie53c4f915b9dec7097bbcb87afad147ec3f9b1aa