SDK / exoplayer

This does currently only happen by chance in replaceStream (called from
enable) if the stream previosly played read until C.TIME_END_OF_SOURCE.

enable already makes all changes done in resetPosition (except resetting
the reading position), so it's less error-prone and makes the intention
clearer if the same code is called from both enable and resetPosition.

The effect of this bug was quite limited because the numerical value
of readingPositionUs was only relevant for periods with changing
durations and server-side inserted ads.

PiperOrigin-RevId: 422300690