Load XML files on disk by default (facebook#1348)

Summary:
Pull Request resolved: facebook#1348

X-link: facebook/react-native#46371

## Internal

Vector drawable image support was added in D59530172 but importing vector drawable asset types was not supported out of the box. It required custom source transformers like the one added in D60021474. This is because Android cannot load vector drawable XML over the network. Vector drawables are compiled by AAPT as part of the build process. Even though Metro can serve XML, it would never load.

## Summary

This adds some minor checks in the `AssetSourceResolver` to only attempt loading XML asset types from disk on the Android platform. XML assets like vector drawables are precompiled and cannot be served over the network by Metro.

## Changelog

[Android] [Added] - Adds support for importing XML assets as images

Reviewed By: javache

Differential Revision: D62302929

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -18,6 +18,7 @@ Object {
       "psd",
       "svg",
       "webp",
+      "xml",
       "m4v",
       "mov",
       "mp4",
@@ -201,6 +202,7 @@ Object {
       "psd",
       "svg",
       "webp",
+      "xml",
       "m4v",
       "mov",
       "mp4",
@@ -384,6 +386,7 @@ Object {
       "psd",
       "svg",
       "webp",
+      "xml",
       "m4v",
       "mov",
       "mp4",
@@ -567,6 +570,7 @@ Object {
       "psd",
       "svg",
       "webp",
+      "xml",
       "m4v",
       "mov",
       "mp4",

packages/metro-config/src/defaults/defaults.js

@@ -25,6 +25,7 @@ exports.assetExts = [
   'psd',
   'svg',
   'webp',
+  'xml',
   // Video formats
   'm4v',
   'mov',