Keep overlay displays in the default power group

- Overlay displays should be added to the default power group, since
  them staying on relies upon the power state of the underlying display.
- Add logging

Bug: 433040478
Bug: 362720020
Flag: EXEMPT minor bug fix
Test: atest KeepScreenOnTests
Test: atest DisplayServiceTests
Change-Id: Id145ce21d154b913e32da9df84ca9d58eab941f6