Refactor idle/low-light state management.

- remove custom idle timeout, now dreams are triggered by system
inactivity
- remove mDozeToDreamLock, which wakes up the device briefly from
dozing, and puts it to dream. Sync with PowerManager on a better
approach.

Test: atest IdleHostViewControllerTest
Bug: 195606657
Fix: 195606657
Change-Id: Ife31527affa3943d5bce4c5ffad35ec33a397528