Since signer is now optional, it makes more sense to pass it in setter
function.
Also fix a problem in command line that if a signer or key source is
not specified correctly, the program should return immediately.
Change-Id: I3be6a4e2ba7bf7b8d5589ac8268390a0fe08a626
Renamed EncryptionKeySource and WidevineEncryptionKeySource to KeySource and
WidevineKeySource respectively.
Replaced Initialize method with FetchKeys.
Added unit tests.
content_id parameter is now binary. Specified in command line with hex string.
Change-Id: I8010ca62d036cefb3a7c105e0eefee25bbf06d61
KongQun Yang
Thomas Inskip