[TIMOB-379] Search bar: only show cancel button when search input has the focus.

Enhancement suggestion: Currently, when cancel is 'true', the cancel button is always displayed. It's disabled if the search input doesn't have the focus.

I believe that a different behavior would create a cleaner UI: Make the cancel button invisible instead of disabled when the search bar doesn't have the focus. On the iPhone platform, the disabled state is still somewhat 'active'-looking. The presence of the disabled button is also unnecessary clutter.