Move BufferHubDefs namespace to ui/BufferHubDefs.h

And move libbufferhubservice off libbufferhub_headers and libpdx_headers
dependency.

Test: build passed, including all the tests under libui and
libbufferhubservice
Bug: 118893702

Change-Id: I8163a7524c6078f9fbb264cf5253d8cbe05dd1c4
8 files changed