Expose an elementary stream source which can be used by an app

### [REQUIRED] Use case description
Add support for MSE EME. Because certain apps may intend to use both MSE EME for certain formats along with the current model. Although the apps can directly use the decoders(decoder interfaces and implement the data injection, it would be good to have the Exo Player support this so that apps can use it even for MSE EME kind of playback.

### Proposed solution
Add elementary stream data source which can inject demuxed audio and video packets. Support decryption of the same when they are found to be encrypted when the content is protected with different DRM schemes.

### Alternatives considered
Not thought about it.