libstagefright: Fix for ANR in streaming during seek

In seek, OMXCodec sets state to FLUSHING and if it
gets EOS during seek then the state will remain in
FLUSHING state. OMXCodec stop will happen only in
ERROR, PAUSING and EXECUTING states. Hence solution
is to set mState to EXECUTING after flushing completed

CRs-Fixed: 664381

Change-Id: Ie6f65175ce66ea6973465b68d31e4f82354ea9a9