bufferstreams: Make BufferOwner Send+Sync

This is important for IPCs to work, since buffers can come in over the
wire, be processed by binder threads, and handed off to
client-controlled worker threads.

Bug: 296099728
Test: atest
Change-Id: I9c32ac17e07456aa6a8e7211c67893a51fc9724c
1 file changed