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)
- 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: Ib962b2cd62c991fe4d6f3b6b27b5d97c9ec653b0

Comments from Slim dev:-
just a small hint dirty flashers will have a huge navbar suddenly on bootup
(at least if you had before changed something on the navbar heights) :p
so reset it to default in settings with the reset button or clean install
one of both :D