Make multiproduct_kati call soong_ui.bash .

This serves to not link parts of soong_ui (and eventually soong_build)
into a separate, weird binary. This is in turn good because they contain
any number of global variables and no one really thought about what
happens when two instances are executing at the same time in the same
address space.

This comes with a slight performance hit: 5 aosp_* projects build 152
seconds instead of 146. I suppose this is a price worth paying for a
clean design?

Test: presubmits.
Change-Id: I5623dcab2290f0fc392dd2ede597b9794a3d2a4e

ui/build/build.go

@@ -238,6 +238,11 @@ func Build(ctx Context, config Config) {
 		ctx.Verboseln("Skipping use of Kati ninja as requested")
 		what = what &^ RunKatiNinja
 	}
+	if config.SkipSoong() {
+		ctx.Verboseln("Skipping use of Soong as requested")
+		what = what &^ RunSoong
+	}
+
 	if config.SkipNinja() {
 		ctx.Verboseln("Skipping Ninja as requested")
 		what = what &^ RunNinja