This is in preparation of supporting entitlement license API, where
common encryption server may return concatenated PSSHs directly.
Refactored ProtectionSystemSpecificInfo into a struct containing
concatenated PSSHs. This will make it easier to pass PSSHs around.
Also, most of the time, users of ProtectionSystemSpecificInfo do
not care what is in PSSH; so moved PSSH box parsing and building out
of ProtectionSystemSpecificInfo.
b/78171767
Change-Id: I1c4d5e7e23efd2f7d4b2b9704378323112e47f00
b5a73fc1d5