kmsalphablending: Send a flush stop event before EOS
kmsalphablending: Send a flush stop event before EOS

Change-Id: I1e67dcbeddc741d29a9b37fd9a392350c85898a6

TribeMedia/kms-elements,TribeMedia/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements