prepare and selectTracks receive the position from which any
loading should start, where-as continueLoading receives the
actual playback position. These are different in the case that
a previous period is still being played out.
Also removed "relative to the start of the period" from prepare
documentation because it couldn't really be relative to anything
else.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172592769
