Android 12.0.0 release 21
Adds a README and canonical example for treble_manifest_split. am: 70e8444410 am: 18065746ff am: efa11f1338

Original change: https://android-review.googlesource.com/c/platform/tools/treble/+/1689267

Change-Id: I3b2bf4c01ef218eefdd18c65dd5dc09f152d1b4b