Use version mutator for NDK

The ndk_api mutator is similar to the version mutator.  Move the
ndk_library ndk_api variations into the version mutator instead,
which will help later when consolidating the stubs handling
between NDK, LLDNK and Apex libraries.

Test: No change to build.ninja or Android-${TARGET_PRODUCT}.mk
Change-Id: I51417cf669265762c15f7289e1dc186d017ef4a9

cc/ndk_sysroot.go

@@ -131,7 +131,7 @@ func (n *ndkSingleton) GenerateBuildActions(ctx android.SingletonContext) {
 		}
 
 		if m, ok := module.(*Module); ok {
-			if installer, ok := m.installer.(*stubDecorator); ok {
+			if installer, ok := m.installer.(*stubDecorator); ok && m.BuildStubs() {
 				if ctx.Config().ExcludeDraftNdkApis() &&
 					installer.properties.Draft {
 					return