VDM API for specifying custom home component.

That component is respected whenever a virtual display is created
by the device and that display supports showing home activities.

Using `VIRTUAL_DISPLAY_FLAG_SHOULD_SHOW_SYSTEM_DECORATIONS` for now
and adding a TODO to reference whatever API lands for creating
displays with home support.

Test: atest VirtualDeviceHomeTest
Fix: 297168328
Change-Id: Iced6d176fac39b7cdf375c40de1898c9090a79a3