Add system_ext support to build_prop module

As some partitions have build.prop under etc/, this change adds
relative_install_path property to build_prop module. Also this change
adds system_ext related Soong variables and system_ext support in
gen_build_prop.py.

Bug: 322090587
Test: build and compare system_ext/etc/build.prop
Change-Id: I416662b8bae09383af0cdd3d8444a5c300006b7b

Android.bp

@@ -141,3 +141,12 @@ build_prop {
         "//packages/modules/Virtualization/build/microdroid",
     ],
 }
+
+build_prop {
+    name: "system_ext-build.prop",
+    stem: "build.prop",
+    system_ext_specific: true,
+    product_config: ":product_config",
+    relative_install_path: "etc", // system_ext/etc/build.prop
+    visibility: ["//visibility:private"],
+}