Don't create SDK variants for native bridge modules

Native bridge modules never need to build against NDK stubs, so
don't create SDK variants.  Also clear the sdk_version property
for modules that don't have SDK variants so that later code doesn't
use it to trigger SDK behaviors.

Test: m checkbuild
Change-Id: I1920fa82e9fab06235f01a62624382efa16cc6e3

cc/ndk_library.go

@@ -354,6 +354,5 @@ func newStubLibrary() *Module {
 func NdkLibraryFactory() android.Module {
 	module := newStubLibrary()
 	android.InitAndroidArchModule(module, android.DeviceSupported, android.MultilibBoth)
-	module.ModuleBase.EnableNativeBridgeSupportByDefault()
 	return module
 }