Use a separate mutex for BLASTBufferQueue in BLASTBufferItemConsumer am: 5c8b18c908 am: 618f49aded am: 12b8cad1e7

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/15728802

Change-Id: I9b788c876664c41837daf8a865af4bff767ec3c4