Add build_prop soong module

build_prop is a soong module generating {partition}/build.prop. It's
ported from build/make/core/sysprop.mk and will replace sysprop.mk in
the future. Currently used only by Android.bp cuttlefish system image.

Bug: 322090587
Bug: 346214958
Test: compare system/build.prop and aosp_cf_x86_64_system-build.prop for
      eng / user / userdebug
Change-Id: Iad76139fe86ec4f582218c185aa15baeacaa38dc

Android.bp

@@ -122,6 +122,7 @@ dexpreopt_systemserver_check {
 }
 
 // buildinfo.prop contains common properties for system/build.prop, like ro.build.version.*
+// TODO(b/322090587): merge this to gen_build_prop.py script.
 buildinfo_prop {
     name: "buildinfo.prop",
 
@@ -141,5 +142,5 @@ all_apex_contributions {
 
 product_config {
     name: "product_config",
-    visibility: ["//visibility:private"],
+    visibility: ["//device/google/cuttlefish/system_image"],
 }