Use exportable=true when reading certificate from db
Use exportable=true when reading certificate from db

[Problem] Key manager allows creating a cert chain from not exportable
certificates.
[Solution] CKMLogic::readCertificateHelper modified to use exportable flag
equal to 'true'.

[Verification] Run ckm-tests-internal -t ENCRYPTION_SCHEME_TEST

Change-Id: Ib13811282eb9d1267c26741a578d8c2111bdecbb

Samsung/ckm,Samsung/ckm,Samsung/ckm