Split ArchMutatorContext out of BaseMutatorContext

Split the context methods that are useful on anything visiting a
module that has arch variants into a separate ArchMutatorContext
for reuse by TranstitionMutators.

Bug: 319288033
Test: builds
Change-Id: Ifdc21983c6c79f22965a49f169812a8cc3ad975b

android/Android.bp

@@ -35,6 +35,7 @@ bootstrap_go_package {
         "api_levels.go",
         "arch.go",
         "arch_list.go",
+        "arch_module_context.go",
         "base_module_context.go",
         "buildinfo_prop.go",
         "config.go",