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