VTS: move output validation after flush to a method

Test: atest VtsHalMediaC2V1_0TargetAudioDecTest
Test: atest VtsHalMediaC2V1_0TargetAudioEncTest
Test: atest VtsHalMediaC2V1_0TargetVideoDecTest
Test: atest VtsHalMediaC2V1_0TargetVideoEncTest

Bug: 151703904
Change-Id: Ie3b71978a082cfe47568fc34b50e8b8f7748a548
6 files changed