Implement duplicate buffer

Allow BufferClient to generate a opaque token in hidl_handle (i.e.
native_handle_t) format that could be used for IBufferHub::import.

The service will keep a mapping from token to client. Token comparison
is handled in BufferHubService.cpp, as it's opaque to the users.

Test: BufferHubBuffer_test (passed)
Bug: 118614157
Change-Id: Ie2b0b650dba90fe2ed2f8091dd88acb9768f261f