Clear MultiAccessUnit SkipCutBuffer

 - Adding clear() to skipCutBuffer when flushing
 - MultiAccessUnitHelper flush will flush all
   frameIndexes within a buffer group and mark
   the frame comple when flagged with DISCARD

Bug: 324224579
Test: Test: atest android.mediav2.cts.CodecDecoderMultiAccessUnitTest

Change-Id: Icf735aab92c019638d8974031e8dcc24299f1a42