Cronet extension project setup

- Add build test to [] so it runs in presubmit
- Tweak README.md instructions
- Use symlinks to make the extension usable in Android
  Studio internally, without taking any special action
- Remove unnecessary dependencies
- Simplify setup instructions in README.md

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129733466

.gitignore

@@ -55,3 +55,9 @@ extensions/flac/src/main/jni/flac
 
 # FFmpeg extension
 extensions/ffmpeg/src/main/jni/ffmpeg
+
+# Cronet extension
+extensions/cronet/jniLibs/*
+!extensions/cronet/jniLibs/README.md
+extensions/cronet/libs/*
+!extensions/cronet/libs/README.md