[NUI] Modify the default action of back navigation

[NUI] Modify the default action of back navigation

Previously, back navigation only pops the peek page.
However, in most cases, application wants to exit the program when back
button is clicked on the first page.
This forces application to customize back navigation to change the
default action of back navigation.

So now, the default action of back navigation has been modified.
If navigator has more than one page, then the peek page is popped by
back navigation.
Otherwise, the current program is exited by back navigation.