Docs for Safe Area Layout Guides result in unsatisfiable constraints

[RickyRomero]

This repo has a test target you can build and run, "TestApplication." The docs list an alternative approach for handling safeAreaLayoutGuide.

If you make the changes listed in that code example to TestApplication and nothing else, it results in unsatisfiable constraints.

2018-06-10 18:39:20.523973-0700 TestApplication[80159:3227789] [LayoutConstraints] Unable to simultaneously satisfy constraints.
	Probably at least one of the constraints in the following list is one you don't want. 
	Try this: 
		(1) look at each constraint and try to figure out which you don't expect; 
		(2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x60c00008f280 UITableView:0x7fd91784fe00.bottom <= UILayoutGuide:0x60c0001af260'UIViewSafeAreaLayoutGuide'.bottom   (active)>",
    "<NSLayoutConstraint:0x60c00008f780 V:[UIView:0x7fd91a801460]-(0)-[KeyboardAdjuster.KeyboardLayoutGuide:0x60c0001c2760'KeyboardLayoutGuide']   (active)>",
    "<NSLayoutConstraint:0x60c00008fb40 UITableView:0x7fd91784fe00.bottom >= KeyboardAdjuster.KeyboardLayoutGuide:0x60c0001c2760'KeyboardLayoutGuide'.top   (active)>",
    "<NSLayoutConstraint:0x60c00008edd0 'UIViewSafeAreaLayoutGuide-bottom' V:[UILayoutGuide:0x60c0001af260'UIViewSafeAreaLayoutGuide']-(34)-|   (active, names: '|':UIView:0x7fd91a801460 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x60c00008f780 V:[UIView:0x7fd91a801460]-(0)-[KeyboardAdjuster.KeyboardLayoutGuide:0x60c0001c2760'KeyboardLayoutGuide']   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

This PR reduces the priority of the top keyboard constraint to produce the intended behavior.