Make setting of accessible attributes more consistent

[jessegreenberg]

Part of #953, there are a few ways now to set accessible attributes (internally to scenery) on an AccessiblePeer. For example, to set the input value, Accessibility.js uses

           for ( var i = 0; i < this.accessibleInstances.length; i++ ) {
              var peer = this.accessibleInstances[ i ].peer;
              peer.onInputValueChange();
            }

But to mark something as checked, Accessibility uses setAttributeToElement

            this.setAccessibleAttribute( 'checked', checked, {
              asProperty: true
            } );

Discussing with @zepumph, we prefer to use setAccessibleAttribute directly to avoid a proliferation of functions on AccessiblePeer for numerous HTML attributes. But if the individual function on AccessiblePeer is doing more work it has more value.