Add DistForGoal "droidcore" for merged_compat_config.xml

Convert the tools/platform-compat/Android.mk into soong build.

Bug: 344489076
Test: m dist
Change-Id: I1e5a744af0005b913d9831d939465af1e2cf825e
diff --git a/java/platform_compat_config.go b/java/platform_compat_config.go
index 99fa092..49756dd 100644
--- a/java/platform_compat_config.go
+++ b/java/platform_compat_config.go
@@ -266,7 +266,7 @@
 
 func (p *platformCompatConfigSingleton) MakeVars(ctx android.MakeVarsContext) {
 	if p.metadata != nil {
-		ctx.Strict("INTERNAL_PLATFORM_MERGED_COMPAT_CONFIG", p.metadata.String())
+		ctx.DistForGoal("droidcore", p.metadata)
 	}
 }