rust: Refactor staticStd to stdLinkage

Instead of returning a boolean, return an enum value to improve
readability and provide greater flexibility for future modifications.

Bug: 168729404
Test: Soong tests pass
Change-Id: Iddcdae8c34be09e476404382e43d1ea5935bae65
diff --git a/rust/rust.go b/rust/rust.go
index 22b81f1..f88b310 100644
--- a/rust/rust.go
+++ b/rust/rust.go
@@ -294,7 +294,7 @@
 	Disabled() bool
 	SetDisabled()
 
-	staticStd(ctx *depsContext) bool
+	stdLinkage(ctx *depsContext) RustLinkage
 }
 
 type exportedFlagsProducer interface {
@@ -997,8 +997,9 @@
 		commonDepVariations = append(commonDepVariations,
 			blueprint.Variation{Mutator: "image", Variation: android.CoreVariation})
 	}
+
 	stdLinkage := "dylib-std"
-	if mod.compiler.staticStd(ctx) {
+	if mod.compiler.stdLinkage(ctx) == RlibLinkage {
 		stdLinkage = "rlib-std"
 	}
 
@@ -1030,7 +1031,7 @@
 		}
 	}
 	if deps.Stdlibs != nil {
-		if mod.compiler.staticStd(ctx) {
+		if mod.compiler.stdLinkage(ctx) == RlibLinkage {
 			actx.AddVariationDependencies(
 				append(commonDepVariations, blueprint.Variation{Mutator: "rust_libraries", Variation: "rlib"}),
 				rlibDepTag, deps.Stdlibs...)