Add multiproduct_kati

This is a replacement for build/tools/kati_all_products.sh using the new
Soong ui/build package. It doesn't even attempt to run ninja, and it can
be configured to run only the product config, or only the product config
and Soong.

For AOSP on my machine:
 -only-config  1.4s
 -only-soong   1m20s
 <none>        13m

Test: multiproduct_kati
Change-Id: Ie3e6e7bdf692e46a8b8eb828f437190f8003500b

cmd/soong_ui/main.go

@@ -48,14 +48,6 @@ func main() {
 		log.Fatalln("The `soong` native UI is not yet available.")
 	}
 
-	// Precondition: the current directory is the top of the source tree
-	if _, err := os.Stat("build/soong/root.bp"); err != nil {
-		if os.IsNotExist(err) {
-			log.Fatalln("soong_ui should run from the root of the source directory: build/soong/root.bp not found")
-		}
-		log.Fatalln("Error verifying tree state:", err)
-	}
-
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()