Support testing is hierarchical. Player.support returns an object
mapping out the support present in various parts of the system.
This creates a shell for Player and DrmEngine, and introduces support
tests for MediaSourceEngine, ManifestParser, and DrmEngine.
This also adjusts the way support checks are done for text types, to
allow for expansion into MP4-embedded text types and other segmented
subtitle types.
Text changes related to #150 and #146
Change-Id: I707f01ba52cba6262ee89bee2c1f28d24aca4655