Minor changes

* new icons with 3D effect as required in Android Iconography Guide
* target SDK version updated to 20
* replaced UI directions left/right by start/end for RTL support
* Show "unnamed calendar"/"unnamed address book" instead of "null" in "Select collections" fragment
* "Add account" button now directly adds DAVdroid account
* .svgz for "How DAVdroid interacts with other components" added to doc