TextDescription: Fix to parse ftab box

Test: atest TimedTextUnitTest -- --enable-module-dynamic-download=true

Bug: 150915309

Change-Id: I4b1c61661948f5613cf6dccbbcf4367bd6a291c7
Merged-In: I4b1c61661948f5613cf6dccbbcf4367bd6a291c7
1 file changed