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
diff --git a/rust/rust.go b/rust/rust.go
index 68e5a5b..6004c68 100644
--- a/rust/rust.go
+++ b/rust/rust.go
@@ -221,6 +221,10 @@
 	return false
 }
 
+func (mod *Module) SplitPerApiLevel() bool {
+	return false
+}
+
 func (mod *Module) ToolchainLibrary() bool {
 	return false
 }