DisplayCutout: device-independent emulation overlay

Adds an option to use dp instead of px when specifying the cutout bounds.
Also centers the coordinate system in the middle, making it easier to specify
the usually centered cutouts.

Also makes the emulated cutout a bit prettier.

Bug: 65689439
Test: adb shell overlay enable com.android.display.cutout.emulation
Change-Id: I3bd16af15f1dad2af204d436abaa35fb9e5ae146