Add BufferHubEventFd to bufferhub system

Now BufferNode will create a new BufferHubEventFd instance during
allocation, BufferHubService will pass that fd to client side, and
BufferHubBuffer will now extract and import it.

User can now access the eventFd by a public function of BufferHubBuffer.

Update BufferHubBuffer_test to check if the two event fds are actually
linked to each other.

Test: VtsHalBufferHubV1_0TargetTest, BufferHubServer_test,
BufferHub_test
Fix: 68770788

Change-Id: I7f6e07d17615d2b45c0e7e086c481292c5798e97