StageFright: SampleTable sync sample search procedure correction

There was a problem with calculation of closest sample in sample table:
it didn't return the closest sample, but always the next one.
This patch corrects this by adjusting this value to the closest,
as it should be.

Change-Id: Ie8fcdcde8754243f6db5d14830e394667b94a9b1
Signed-off-by: Dmytro Dubovyk <dmytro.dubovyk@ti.com>