Add a service-unique ID to BufferNode

All the BufferNode allocated by bufferhub hwservice now will have a
unique ID generated and managed by the UniqueIdGenerator.

The idGenerator is placed in global static namespace because 1)
therefore it is process-unique 2) BufferNode could still access it
without a reference to the service instance.

Added UniqueIdGenerator_test.

Test: UniqueIdGenerator_test, BufferNode_test, buffer_hub-test,
BufferHubBuffer_test passed.
Fix: 118844348
Change-Id: I9c9adebab3af7b9de71dbe8728d3f24ed231338d