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Update README for errorprone and add owners. Use a single jar to make
maintenance easier and to pave the way to updates in the future.

BUG=383646

Change-Id: Ib9e2900141baab73515c957bb274b7568ef3102a
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/700856
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506526}