Cut the multiproduct_kati -> soong-ui-build dep.

This is done by moving SetupSignals() to its own little package.

There are a number of tiny little utility packages for soong_ui we might
be better of merging, but that's for another change (maybe)

Test: Presubmits.
Change-Id: I07b0ca98bfb8884ef4223d665e632183b9896a0d

ui/build/Android.bp

@@ -61,7 +61,6 @@ bootstrap_go_package {
         "proc_sync.go",
         "rbe.go",
         "sandbox_config.go",
-        "signal.go",
         "soong.go",
         "test_build.go",
         "upload.go",