Core changes to support desktop mode option

Internal system setting for enabling desktop mode prototype behavior.
This setting is triggered initially by a developer tile.
WM Shell in SystemUI will be listening for changes to this setting and
update the windowing mode respectively.

Bug: 241944030
Test: atest SettingsBackupTest
Change-Id: Ie7172d602b55e079b1fac4c83886f00cbb9baa18