Fix ampersand handling in Span class

Fix ampersand handling in Span class

Ampersand character ('&') has not been handled correctly, causing the
text handled by Span class to display solely the initial part, up to the
first '&' character.
This bug has been spotted in Xamarin.Samples.TipCalc application, where
the displayed label's caption stated "Food ", but pre-programmed string
was "Food & Drink".
This commit makes the Span class handle the '&' character correctly,
which fixes that bug.

Change-Id: I0e17d39fbfae6a5ef5eea585a5229c4a5b0cca7e
Signed-off-by: Adam Szczerbiak <a.szczerbiak@samsung.com>