rename background image for all build types

relates to: #8898 (closed)

Currently the default background image for different contexts (Navigation Drawer, Splash Screen, Main Screen [=EipFragment], Provider Detail Screen, Login) is called ic_colorsquare. Instead of describing the the content (a colorsquare) the reference to the background image should be renamed according to the context, e.g. background_main, background_drawer etc.. This would allow us to have different background images in the main screen, navigation drawer, splash screen and login/provider detail screen for a custom branded app.