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
diff --git a/android/config.go b/android/config.go
index 40a73d6..d27848d 100644
--- a/android/config.go
+++ b/android/config.go
@@ -2086,6 +2086,10 @@
 	return PathsForSource(ctx, c.productVariables.SystemPropFiles)
 }
 
+func (c *config) SystemExtPropFiles(ctx PathContext) Paths {
+	return PathsForSource(ctx, c.productVariables.SystemExtPropFiles)
+}
+
 func (c *config) EnableUffdGc() string {
 	return String(c.productVariables.EnableUffdGc)
 }