Browser: disable incognito mode

Chromium Webview does not support incognito mode which will result
in force close, disabling for now.

Change-Id: I501a609f94c115b9a9d1755ef0ea73eb9b29c823

Conflicts:

	res/layout-land/nav_screen.xml
	res/layout/nav_screen.xml
	src/com/android/browser/NavScreen.java
	src/com/android/browser/PhoneUi.java