Introduce the SystemUIComposeGallery app (1/2)

This CL introduces the SystemUIComposeGallery application which will be
used to showcase our different Compose UIs and themes. This app will
also be used for rapid UI development so that engineers writing a
Compose UI won't have to build SystemUI at all when working purely on
their UI.

See b/231131244#comment2 for an example on how this app will look like
after this CL.

Bug: 231131244
Test: Manual
Change-Id: I9daa750e4dc5ffba2f668eb0a149a5b6444f2ccf