Don't make SplitPerApiLevel imply UseSdk

UseSdk was returning true when SplitPerApiLevel returned true,
which was causing the platform variant of SplitPerApiLevel
module to compile against the SDK.  Check SplitPerApiLevel
separately in the sdkMutator instead.

Test: m checkbuild
Change-Id: I0ae667d48a3b7b96709a6cad8e8ea9701659fc2a

cc/sdk.go

@@ -32,11 +32,11 @@ func sdkMutator(ctx android.BottomUpMutatorContext) {
 	switch m := ctx.Module().(type) {
 	case LinkableInterface:
 		if m.AlwaysSdk() {
-			if !m.UseSdk() {
+			if !m.UseSdk() && !m.SplitPerApiLevel() {
 				ctx.ModuleErrorf("UseSdk() must return true when AlwaysSdk is set, did the factory forget to set Sdk_version?")
 			}
 			ctx.CreateVariations("sdk")
-		} else if m.UseSdk() {
+		} else if m.UseSdk() || m.SplitPerApiLevel() {
 			modules := ctx.CreateVariations("", "sdk")
 			modules[0].(*Module).Properties.Sdk_version = nil
 			modules[1].(*Module).Properties.IsSdkVariant = true