libstagefright: A fix to ensure we can get the closest sync frame to the seek point

JB adopts binary search for the closest sync frame
to the seek point. We need to have the left end
subtract 1 to meet the need.

Change-Id: I53b86d208bd291b02d360d039d2dfeeb1da88ae3
Author: Tianmi Chen <tianmi.chen@intel.com>
Signed-off-by: Tianmi Chen <tianmi.chen@intel.com>
Signed-off-by: Dan Liang <dan.liang@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 55341