ExoPlayerImpl.prepare() replaces the timeline with an empty timeline. After this
happens, MSG_SOURCE_INFO_REFRESHED could be handled on the main thread and
could relate to the old source, so the player could expose a stale timeline.
Count pending prepares in ExoPlayerImpl so that source info refreshes can be
suppressed until preparation actually completes.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152496255
