Part of fix for double spins & faster startup

This change alters the way that the media player
interacts with the Widevine adaptive streaming buffer
logic.  It eliminates the reliance on cached buffer
duration to determine pause/play states and instead
only generates buffering events when the widevine
library is not producing data (i.e. when it is
buffering).  This eliminates unnecessary pause and
rebuffer cycles, reducing startup time and the
frequency and duration of spinners.

Multi-repo commit, depends on related changes in vendor/widevine

Change-Id: I387db2decd83841775a8351bddf5a6381fe4104f
related-to-bug:6503294
related-to-bug:6463780