bufferhubd: Add DetachedBufferChannel

1/ Introduces four new BufferHub RPC operations.
2/ Implements the ProducerBufferDetach with test case.
3/ Also fixes a bug that Hangup signal wasn't propogated to the client
   when bufferhubd closes a producer channel.

Bug: 38137191
Bug: 70046255
Bug: 70912269
Test: buffer_hub-test
Change-Id: Ia2ba0e95abd3c1b960670c505c6fdb9c9de3a6dd
diff --git a/libs/vr/libbufferhub/include/private/dvr/buffer_hub_client.h b/libs/vr/libbufferhub/include/private/dvr/buffer_hub_client.h
index c791250..32448a1 100644
--- a/libs/vr/libbufferhub/include/private/dvr/buffer_hub_client.h
+++ b/libs/vr/libbufferhub/include/private/dvr/buffer_hub_client.h
@@ -217,6 +217,14 @@
   // succeeded, or a negative errno code if local error check fails.
   int GainAsync(DvrNativeBufferMetadata* out_meta, LocalHandle* out_fence);
 
+  // Detaches a ProducerBuffer from an existing producer/consumer set. Can only
+  // be called when a producer buffer has exclusive access to the buffer (i.e.
+  // in the gain'ed state). On the successful return of the IPC call, a new
+  // LocalChannelHandle representing a detached buffer will be returned and all
+  // existing producer and consumer channels will be closed. Further IPCs
+  // towards those channels will return error.
+  Status<LocalChannelHandle> Detach();
+
  private:
   friend BASE;