Use pressBack

pressBack blocks until back is pressed. This is much better than the old
implementation which returns immediately.
Dump without disconnecting the current session

The old way of viewing the page source broke the current uiautomator session. A
lot of effort went into detecting a failed session and then restarting. It turns
out we can access the source from within a session.
Use uiautomator pressKeyCode

The adb solution doesn't wait until the key has finished which causes problems
in tests. uiautomator's built in pressKeyCode is much better.
Merge pull request #925 from bootstraponline/fix_back

Use pressBack