SDK / exoplayer

Include startMediaTime in media position checkpoints.

We currently apply new parameter checkpoints from an absolute media
time and then substract the current media start time again to retrieve
the media time offset for this playback parameter checkpoint.

However, the media start time may change when unexpected discontinuities
happen (the start time doesn't actually change, but we change it to
correct for this discontinuity). This then invalidates the absolute
media time in the playback parameter checkpoints (which should have been
corrected as well).

Avoid this problem by also only applying the new start position
from the checkpoint. We don't have to save the start position anymore
because it will cancel itself out.

Also add some documentation and code clarification for improved
readability.

PiperOrigin-RevId: 291923069