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

scripts/Android.bp

@@ -299,6 +299,12 @@ python_binary_host {
     ],
 }
 
+python_binary_host {
+    name: "gen_build_prop",
+    main: "gen_build_prop.py",
+    srcs: ["gen_build_prop.py"],
+}
+
 python_binary_host {
     name: "buildinfo",
     main: "buildinfo.py",