fb: Handle navbar heights correct on dpi changes + more (1/2)
Till now we set the height in px in the settings. Which is a problem
especially on multiDPI roms like we are when the user afterwards changes
the dpi due that it isn't calculated correct anymore on the new dpi.

This commit does
- save the dp value in provider and calculate the real px in fb
- clean up the settings stuff for it + do it a bit nicer (only settings part
  where we do not set the value for the listview)
- add Converter class for common methods we always use....till now only px->dp
  and dp->px method

Change-Id: I413634fc433e0501eba69db2a463fac231e22f72