Use empty string for core image variant

Use the empty string for the core image variant so that modules
added to imageMutator do not change their build directory.

Bug: 142286466
Test: m checkbuild
Change-Id: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
Merged-In: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
(cherry picked from commit 72d685ee7f45e5393be44ae4159edf083ac918de)

rust/rust_test.go

@@ -245,7 +245,7 @@ func TestNoStdlibs(t *testing.T) {
 			srcs: ["foo.rs"],
                         no_stdlibs: true,
 		}`)
-	module := ctx.ModuleForTests("fizz-buzz", "android_arm64_armv8-a_core").Module().(*Module)
+	module := ctx.ModuleForTests("fizz-buzz", "android_arm64_armv8-a").Module().(*Module)
 
 	if android.InList("libstd", module.Properties.AndroidMkDylibs) {
 		t.Errorf("no_stdlibs did not suppress dependency on libstd")