Squashed 'external/devlib/' changes from 492284f46..678822f9e

678822f9e utils/misc: Cleanup check_output()
be734140b utils/android: Make AdbConnection.active_connections thread safe
b988e245d utils/android: Fix AdbConnection.adb_root()
b7ef2dc2e devlib.target: Fix AndroidTarget unpickle

git-subtree-dir: external/devlib
git-subtree-split: 678822f9e46b33fe6bb61f7731864134e9e5432a