MediaPlayer2 DRM: API improvements

* Document how to build valid DrmPreparationInfo
* Add getters for DrmPreparationInfo
* Make DrmEventCallback an abstract class
* Document set/clearDrmEventCallback are synchronous

Bug: 128548364
Test: build
Change-Id: Iecc2a8bcd410ea729ac9fd4c2b3f1428fc4b5e7d
2 files changed