Use only the first 16 bytes of the asset key.
Bug: 18003864

Change-Id: I65b08c4766a4c2ab08b09660b1540630387ab36c
Fix two packager crash bug with invalid arguments

Fix b/18005827 Packager crashes in debug mode with widevine decryption
enabled given a non-existing file.

Fix b/18005632 Packager crashes with invalid fixed_key_encryption
parameters.

Also cleans up some code.

Change-Id: I097f5c8dc113eb6d33b42b19a4bf5de125c47c30