Add sleep after restarting usb ports and before checking for devices.

Need to wait a couple seconds after restarting usb before doing 'adb devices',
or the devices do not show up.

E.g. https://chromegw.corp.google.com/i/clank/builders/s3-sharded-official-perf-
noflash-clankium/builds/3572/steps/device_status_check/logs/stdio

BUG=299891
TBR=ilevy@chromium.org

Review URL: https://codereview.chromium.org/38483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231310 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231310