adb: add `adb reconnect offline` to reconnect offline devices.

Add a command to reconnect offline/unauthorized devices, mainly for use
with the inotify-monitoring of vendor key directories added by 2e671202.

Bug: http://b/29273531
Test: manually tested with a sailfish + copying vendor keys
Change-Id: If34cccee4ae553ada65d128b57d03cba8c0d7c46