mediaplayer: Add MediaClock component and use it in NuPlayerRenderer.

Also fix some racing conditions, remove unused code in NuPlayerRenderer.

This obsoletes https://googleplex-android-review.git.corp.google.com/#/c/609880/.

Bug: 19196501
Change-Id: I0eebeb573ce7229dd8bacecd0e1c5d6f5634a72f
5 files changed