Use sp<> on creating BufferHubBinderService

The original code implicitly calls the constructor of sp<> in
addService() function call, which is not good for readability. Replace
it with sp<> could improve readability and avoid possible memory leak if
the implementation detail of sp<> changes.

Test: atest buffer_hub_binder_service-test (passed)
Bug: 115429751
Change-Id: Ice304a638b8d353a3b229e48c17ecd6f32a66f37