We can acknoledge seeks before preparation finished immediately now,
because ExoPlayerImpl won't leave the masking state until the first prepare
operation is processed.
As a side effect, it also cleans up the responsibility of the callbacks.
Prepares are always acknowledged with a SOURCE_INFO_REFRESHED, while seeks
are always acknowledged with a SEEK_ACK.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177144089
