ENYO-3927: Prevent pressed state from appearing for disabled button.

Issue
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Disabled buttons are still displaying the pressed state when tapped; only seems to occur on mobile devices.

Fix
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We detect if the button is disabled and refrain from adding/removing the `pressed` class as necessary.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>