Fixed navigating using 'Backspace' key

Fixed navigating using 'Backspace' key

[Issue]    https://bugs.tizen.org/jira/browse/TT-238
[Problem]  WebPageUI is closed when 'Backspace' button is used in URIEntry.
[Solution] Added condition in onBackPressed function that prevents
           closing WebPageUI when cursor if URIEntry is focused.
[Verify]   1. Tab Manager > New Tab > press 'Backspace' key > Obs
           Quick access should be closed.
           2. Load a web page > Set focus on URIEntry > press 'Backspace'
           WebPageUI should not be closed.
           3. Tab Manager > New Incognito tab > Press 'Backspace' key > Obs
           Browser shoud not crash

Change-Id: I0fb51f949ced56641d3f8f57b053d0c0e1e339c8