DisplayManagerGlobal: fix native refresh rate callback registration

When AChoreographer is the only callback registered with
DisplayManagerGlobal and there are no other display listeners,
DisplayManagerGlobal incorrectly skips callback registration with DMS.

Bug: 193945763
Test: atest DisplayManagerGlobalTest
Change-Id: I4b737fa0a91541fe331edcf3454724200e2db971