Bugfix: Seek on SRT external track

o Need to return closest upcoming subtitle when seeking time is within
gap ranges.
o Manually cherry-picked from GTV change.
(commit e026a83dc88888b42da77eca739b8f09a0cd6dae)

related-to-bug: 6796228
Change-Id: I02b7718a432d6b2f5575fa1a1a42bda6f04aa25b
4 files changed