This removes reliance on MediaKeys.isTypeSupported and instead uses the
supportedConfigurations parameter of requestMediaKeySystemAccess.
A lot of refactoring went into this effort, particularly around
StreamVideoSource, StreamSetInfo, and StreamInfoProcessor.
Closes#2, b/19354928.
Change-Id: Ibf81da909b8207b5babaec5ef0422dd1293d130d