Add logging to screenonunblocker

- Add logging after windowmanager call resolves so that we can know the
  duration between "Blocking screen on/off..." and the end of the WM
  call. So that we can identify if / where any delays pop up

Bug: 324374971
Flag: EXEMPT solely a logging addition
Test: atest DisplayManagerServiceTests, logcat
Change-Id: I021c5531184e66de7afe56d3aac503d5a4d2f648