SDK / exoplayer

Robustness fix to make sure we ignore tags with known names, but
which are nested inside of unknown tags. For example we don't want
to parse the third period in:

<Period>...</Period>
<Period>...</Period>
<CustomTag>
  <Period>...</Period>
</CustomTag>

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217101630