Use LoadHook to create modules in java_sdk_library

Creating new modules in a mutator is dangerous, as other mutators
that need to see the new modules may already have run, in this case
the prebuilts mutator.  Move SdkLibraryMutator to a LoadHook instead.
Also moves registering the LoadHook mutator to testing.go so it is
registered for all tests.

Test: m checkbuild
Change-Id: I08bd76a0e6205d2ca27861058067a1562c339eed

android/hooks.go

@@ -123,6 +123,10 @@ type hooks struct {
 	install []func(InstallHookContext)
 }
 
+func registerLoadHookMutator(ctx RegisterMutatorsContext) {
+	ctx.TopDown("load_hooks", LoadHookMutator).Parallel()
+}
+
 func LoadHookMutator(ctx TopDownMutatorContext) {
 	if m, ok := ctx.Module().(Module); ok {
 		// Cast through *androidTopDownMutatorContext because AppendProperties is implemented