Adds SystemUiProxy / SystemUiProxyClient

This is a single interface for interacting with ScreenshotProxyService,
which should be renamed prioxy/SystemUiProxyService.

The other direct usages of the service through ServiceConnector
will be consolidated and this interface should be the standard
API.

Bug: 327613051
Test: compile
Flag: None; not used yet
Change-Id: Iace58c81b5fb5bbcb46d27bd2415be550b7c2bb9