Fix direct/offload thread output devices into MSD

Without this change, direct/offload threads show
"Output devices:  (Empty device types)". Empty device
causes timestamp correction to always be disabled for
output threads requiring timestamp correction.

Test: check direct/offload output into MSD dumpsys output devices
      atest audiopolicy_tests
Bug: 159948389
Change-Id: I7006c9da2f0a228dd49ce0297c30035fef983090