NuPlayer: Buffering improvements for http progressive playback

Added changes for
* Buffering preroll till high watermark in preparing state
* Reduce buffering poll delay to 100ms from 1sec in underrun

CRs-Fixed: 776424
Change-Id: Ide155fd5af8786978532ecbd5970bc0871c55186
(cherry picked from commit 215134952980d10633966fd04f96c0f69b24637b)