Implement importBuffer for BufferHubService

Allow user importBuffer by a previously generated handle from
IBufferClient::duplicate.

Passing a nullptr or invalid token to the service will get a nullptr
IBufferClient and BufferHubStatus::INVALID_TOKEN.

Test: BufferHubBuffer_test (passed)
Change-Id: I0aa969a9706e42039ac851acb991b3aceb924c27
Fix: 118614157
5 files changed