NuPlayer: drop temporal-layers to achieve smooth fast-play

For multi-layer-encoded content (with hierarchical referencing),
drop input-frames from selective enhancement layers while
playing back at faster speeds.
This scheme avoids the decoder from decoding *all* the frames,
most of which, would be dropped by the renderer.

Bug: 27596987
Change-Id: I824c699f3c2b80bb516c1b44493eda72239291ac