- Update old comments in sdk.atree.
- Add README.md with instructions on building sdk.
Test: lunch sdk && m -j sdk
Test: Unzipped and extracted platforms/android-11/optional/android.car.jar
and verified it with the latest android.car-stubs.jar from
http://ab/.
Bug: 162442020
Change-Id: I9796fbcecfef5178915008b49d81cda996c66fa2
