KeySource is now an abstract class that is implemented in two
types WidevineKeySource and FixedKeySource. This also updates
fixed key encryption to use v1 PSSH boxes with the common system
ID.
This also updates the --pssh argument. Now it accepts full PSSH
boxes rather than data. Multiple PSSH boxes can be concatenated
together.
Issue #88
Change-Id: I4bd0290f6d8b965f7d118f075c96e0f267d7e831
KongQun Yang
Jacob Trimble