Add importBuffer to BufferHubBinderService

Now you can create a new IBufferClient using token generated by
duplicate(). The new client will be linked to the same node in the
backend.

Import an unexisting token will return PERMISSION_DENIED, while import a
gone buffer will get DEAD_OBJECT.

Test: atest buffer_hub_binder_service-test
Bug: 116681016
Change-Id: Ib07fd160dad98bcaed017b3efa648863b70aa142