DisplayManagerInternal: add cancelBlockScreenOn for DisplayOffloadSession

- Add a cancelBlockScreenOn method in DisplayOffloadSession
- Add test
- Minor test fixes

Flag: com.android.server.display.feature.flags.offload_session_cancel_block_screen_on
Test: atest DisplayOffloadSessionImplTest
Test: atest DisplayPowerControllerTest
Bug: 346420374
Bug: 347415946
Change-Id: Ic043208968bf516a58420e1e293c663d44061782