Add support for consumer queue initial import and hangup.

- Add support for consumer queues to import buffers that are created
  before the consumer queue is created, making multi-consumer queue
  patterns possible. This is essential for VrFlinger operation.
- Add support for notifying consumer queues when the producer queue
  hangs up.
- Correct the epoll event loop to check for hangups even when buffers
  are available.
- Add method to retrieve the event fd from a queue.
- Add trace logging and minor cleanup.
- Improve bufferhubd dump state output.

Bug: 36401174
Test: build; bufferhub tests pass.
Change-Id: Idd6f38a3341c048192062734e288d11de48bc4d4
diff --git a/services/vr/bufferhubd/buffer_hub.cpp b/services/vr/bufferhubd/buffer_hub.cpp
index 8093b6b..debcc73 100644
--- a/services/vr/bufferhubd/buffer_hub.cpp
+++ b/services/vr/bufferhubd/buffer_hub.cpp
@@ -132,7 +132,7 @@
   stream << std::endl;
   stream << "Active Producer Queues:\n";
   stream << std::right << std::setw(6) << "Id";
-  stream << std::right << std::setw(12) << " Allocated";
+  stream << std::right << std::setw(12) << " Capacity";
   stream << std::right << std::setw(12) << " Consumers";
   stream << " UsageSetMask";
   stream << " UsageClearMask";