Add more comments to arch.go

Make sure every exported function or type has a godoc comment.

Also makes minor changes like unexporting functions that are not used
outside the package and fixing minor style warnings.

Bug: 173449605
Test: m checkbuild
Change-Id: I533a595d02035aae8b2b603590be639826d2d4c8
diff --git a/android/defaults.go b/android/defaults.go
index eb013d7..44753ce 100644
--- a/android/defaults.go
+++ b/android/defaults.go
@@ -183,7 +183,7 @@
 
 	initAndroidModuleBase(module)
 	initProductVariableModule(module)
-	InitArchModule(module)
+	initArchModule(module)
 	InitDefaultableModule(module)
 
 	// Add properties that will not have defaults applied to them.