Fix build_prop module to use partition() instead of PartitionTag()

PartitionTag() returns the partition the module is installed in.
We don't want this because a system_ext_specific module could be
installed in the system partition if system_ext partition doesn't
exist.
Instead we want system_ext specific modules to always refer to
SystemExtPropFiles().

Bug: 359624775
Bug: 322090587
Test: build aosp_arm64 and verify system/system_ext/etc/build.prop
Change-Id: Ie67ec9c5e5a07e781515e0aafb42122f72447cf3
diff --git a/android/build_prop.go b/android/build_prop.go
index b127755..13d59f9 100644
--- a/android/build_prop.go
+++ b/android/build_prop.go
@@ -56,7 +56,7 @@
 }
 
 func (p *buildPropModule) propFiles(ctx ModuleContext) Paths {
-	partition := p.PartitionTag(ctx.DeviceConfig())
+	partition := p.partition(ctx.DeviceConfig())
 	if partition == "system" {
 		return ctx.Config().SystemPropFiles(ctx)
 	} else if partition == "system_ext" {