libstagefright: Fix crash during seek

OMXCodec state is changed to EXECUTING when flush
is complete. Update assert condition to check
EXECUTING state instead of FLUSHING.

CRs-Fixed: 607762
Change-Id: I12b3eedfc83e00578d70af3ef903c5a26ccde3eb
Merge "libstagefright: Fix crash during seek" into android-4.4