Add wallpaper dimming SystemAPI and implementation for handling dimming

set by multiple applications.

Handle lock screen dimming regardless if it shares the same wallpaper
with the system to apply composite keyguard + additional dimming.

Test: manual testing and unit tests
Bug: 202841384
Change-Id: Iabd665e9a8e377185b123494eac2198a4436b0c6
CTS-Coverage-Bug: 213426781