SDK / exoplayer

Decode-only video frames (needed when the frame at / first frame after the
clipping start is not a key frame) need to be decoded but not passed to
the frame processor chain or encoder.

The clipping start offset needs to be removed from the frame timestamps
in the passthrough and video pipelines.
There are no changes needed for this in the audio pipeline, as it doesn't
use the input timestamps -- it uses its own timestamps derived from the
buffer sizes instead.

Also add demo option to try this out.

#minor-release

PiperOrigin-RevId: 451353609