Set aria-selected="false" on deselected items

[JanMiksovsky]

As @giltza noted in the original issue on paper-tabs, the aria-selected attribute should be set to "false" for deselected items, not removed.

This PR handles two situations: 1) ensures that all items start out with aria-selected="false" by default, 2) sets aria-selected="false" when a individual item is explicitly deselected.

I've tested this fix under the indicated Windows screen reader, NVDA, and confirmed that paper-tabs now works as expected:

• When using the arrow keys to navigate horizontally among paper-tabs, the name of each tab is announced.
• The screen reader will only announce that the tab is selected if the tab is actually selected. (Previously, the screen reader would announce that the tab was selected, even if it was not, because unselected tabs were missing the aria-selected attribute.)

[cdata]

One underscore indicates protected for us (two indicates private). This technically constitutes a breaking API change. If we can verify it's not being overridden in elements we control that depend on this behavior, it's probably fine..

[JanMiksovsky]

As far as I can see, the only references to _resetTabindices are in iron-menu-behavior.html itself. This also seems like an unusual function for someone to override.

If you'd prefer to play it safe, I could preserve the _resetTabindices name, by either: 1) using the old name for the new function (which does more than reset tab indices) or, 2) creating a new _resetItemAttributes that calls both the old _resetTabindicies and does the new work.