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/Android.bp b/android/Android.bp
index 26605a7..66d361e 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -15,6 +15,7 @@
         "apex.go",
         "api_levels.go",
         "arch.go",
+        "arch_list.go",
         "bazel_handler.go",
         "config.go",
         "csuite_config.go",