Merge of https://widevine-internal-review.git.corp.google.com/c/cdm/+/206431 Message to be signed by CAST funciton is supposed to be in a certain format: "constant prefix + SHA1(message)". Some of our current CAST tests uses random message which break this specification. This fixes the input message. Test: Cast tests with run_fake_l1_tests Bug: 359893908 Change-Id: I6b318d749971d837f13daa7b147313e8e0b1d3d0