ngAria disables tabbing to a radio set when no default value selected

Overview of the Issue - When a radio set is configured with no default selected value (ie: the value starts out as undefined), all the radio buttons will have their tabindex set to -1, making it impossible to tab to the radio set.
Motivation for or Use Case - This makes it difficult to use a normal radio set for focus when you don't want to preselect the value for the user.
Angular Version - 1.4.1
Browsers and Operating System - Tested on FF39 on Mac OS X 10.10.4
Reproduce the Error - View on Plunkr
Related Issues - A partial attempt to fix this was issue ngAria - incorrect "aria-checked" and "tabindex" applied to checkbox/radio #10212
Suggest a Fix - I'm not sure what the right solution here is. Some ideas:
- I don't know what the purpose of setting the tabindex to -1 is, if it's not critical, it probably should be removed.
- Another option would be to find the entire set of radios with a matching name, and if none are selected, then all should be active, otherwise only the selected on is active.

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