Use a separate mutex for BLASTBufferQueue in BLASTBufferItemConsumer am: 5c8b18c908 am: a3970b8d21

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

Change-Id: I212355602594d56d47e3a1a2adfe81aa8ca57c78