SDK / exoplayer

This fixes a very specific case where the data read has non-cached gaps
and a read-only CDS switches to read from upstream in a gap then the
cached data is deleted. When the CDS reaches the end of the gap, it
tries to open the next source. As there is no cached data, it tries to
continue with the already opened upstream data source but as it reached
end of the gap range, the code starts looping.

Also fixes infinite lock which occurs when in the previous case CDS isn't
readonly. It locks the content while filling the gap in the cache. At the
end of the gap, as the following data is deleted it tries to lock the
content for writing but the content is already locked by itself.

The last fix is preventing removal of CachedContent entry from
CachedContentIndex while associated key is locked.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182595426