Desktop screenshots: Dim the region outside partial screenshot

Before taking a partial screenshot, the entire screen should dim. After
draging a region box, only the region outside the region box should dim.

Screenshot: http://go/scrcast/NTg0MDg0NzI2NTMzMzI0OHxhMjllNmZiZS1kMg

Bug: 423962433
Test: Manually on DUT
Flag: com.android.systemui.large_screen_screencapture
Change-Id: I34dcf10c805562c3eca5c5897a13dabb8b5ca2f7