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/testing.go

@@ -37,6 +37,8 @@ func NewTestContext() *TestContext {
 
 	ctx.SetNameInterface(nameResolver)
 
+	ctx.preArch = append(ctx.preArch, registerLoadHookMutator)
+
 	ctx.postDeps = append(ctx.postDeps, registerPathDepsMutator)
 
 	return ctx