SDK / exoplayer

This change constrains the use of sample state member variables to
writeSampleData, finishWriteSampleData and resetWriteSampleData.
Using them elsewhere gets increasingly confusing when considering
features like lacing in full blocks. For example sampleBytesWritten
cannot be used when calling commitSampleToOutput in this case
because we need to write the sample data for multiple samples
before we commit any of them.

Issue: #3026
PiperOrigin-RevId: 284541942