Add crypto_period_seconds to Widevine key request
When using key rotation with Widevine DRM, a key server has to know
the duration of crypto period to relate generated keys to the media
playback time. This helps the server to provide relevant keys to
a client during license request.
Closes#544.
To allow generating Widevine / Playready PSSHs if the corresponding
PSSH generator is specified.
Note that Key Rotation with RawKeySource is designed to be used for
testing only.
Change-Id: Icaf9e74955c082a7b000bd6a08f4561f2e01a2e2