SDK / exoplayer

This adds a parameter to configure a maximum buffer size in bytes. If left
at its default of C.LENGTH_UNSET, the target buffer is determined using a
overridable method based on the track selection. Also adding a parameter
to decide whether to prioritize time or size constraints.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179048554