Settings: Handle navbar heights correct on dpi changes + more (2/2)

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)
- as well this commit disables movable navbar for tablets (we may will enable
  it in future but it needs a lot more work to have it on tablets correct working)
- add Converter class for common methods we always use....till now only px->dp
  and dp->px method

Change-Id: I7320ea0578a10806883a45369dd628f2c51958fa
[2/2] settings: Change density on the fly

Change-Id: I13cd6ed215ff0f81fd418723fe11be9c89556d78