Move global buffer ops to VR display service.

The original home for these ops was the VR display manager service,
which is a protected singleton service that may only have one client.
Since more than one service needs to create global buffers, move these
ops to the display service. They are already protected by permission
checks.

Bug: 62424911
Test: dvr_api-test passes
Change-Id: Ia2f57fdf8a5258b52a652935d160e90db0f1cf9e