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
fb: restore navbar compatibility with several xposed modules
I am tired of the complains....

Xposed use the emulator overwrite to enable/disable navbar...this results
into showing up navbar on reboot even if the user has set it to off in settings.

This fixes the issue and still keeps for emulator tests the basic overwrite functionallity
alive as long the provider value was not set.

We may should reflect that in settings as well.

Change-Id: I99f04da8110bbcfcdba62604610d1f4330aea4d0