Assume *.ktx assets are packaged as Android drawables

Summary:
Updates React Native's asset runtime to look for `*.ktx` assets under `drawable-*` on Android.

This by itself does **not**:
1. Add runtime support for loading KTX images.
2. Update the OSS packager to actually package KTX files as drawables. (This will need to happen [here](https://github.com/react-native-community/cli/blob/cddb0c3fb06ef949ea689221c6167147af40e37f/packages/cli-plugin-metro/src/commands/bundle/assetPathUtils.ts#L39-L46).)

Changelog:

[Android][Changed] Assume *.ktx assets are packaged as Android drawables

Reviewed By: javache

Differential Revision: D32597214

fbshipit-source-id: 25007c1e0eba769ce9e484fa20da5f26890eb38d

packages/assets/path-support.js

@@ -44,6 +44,7 @@ const drawableFileTypes = new Set([
   'gif',
   'jpeg',
   'jpg',
+  'ktx',
   'png',
   'svg',
   'webp',