ENYO-3889: Prevent unintended focusing of input field after tap.

Issue
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On iOS devices, when loading one of the AJAX samples under Enyo Core, the second input field automatically receives focus. This is caused by an iOS-specific issue where the location of the tap that loads the sample propagates forward to the loaded sample, which happens to be the location of the second input field.

Fix
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We call `preventDefault` on the navigation tap event.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>