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