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/variable.go b/android/variable.go
index a715e0e..c9b29f1 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -508,7 +508,8 @@
 
 	ArtTargetIncludeDebugBuild *bool `json:",omitempty"`
 
-	SystemPropFiles []string `json:",omitempty"`
+	SystemPropFiles    []string `json:",omitempty"`
+	SystemExtPropFiles []string `json:",omitempty"`
 
 	EnableUffdGc *string `json:",omitempty"`
 }