Add native full immersive mode (option to ignore display cutout)

Decription copied from https://forum.xda-developers.com/showpost.php?p=83060959&postcount=561

I've found a workaround to enable full immersive mode in all apps or on an app-by-app basis. This workaround has two downsides: The camera hole covers the normal status bar as well (the battery value in percent is hidden) and it requires a magisk module which can be a security risk. Here's how to do it:

• Open Magisk, select "Downloads" in the side menu, search for "None Display Cutout" and install it
• go to Settings>System>Developer Options>Display Cutout and select the new option "No display cutout - MlgmXyysd"
• install any app which can control immersive mode (I like "System UI Tuner") to enable immersive mode or enable it only for some apps
• To partially fix the statusbar go to Settings>System>Statusbar and enable "battery level in percent" next to the symbol
• With root alignment can be further improved by cpadjsutor. For me the ideal offset is 16

It's a bit cumbersome, but chances are this feature will be integrated into a future version of the ROM

Repo of None Display Cutout: https://github.com/Magisk-Modules-Repo/noneDisplayCutout

[cryptomilk]

This is a copy of e.g. frameworks/base/packages/overlays/DisplayCutoutEmulation* with some changes to make it None. We could create a copy and ship it in the tree or suggest it upstream.