SystemUI: fix default navigation bar transparency value

On a fresh flash the navigation bar was completely transparent.
Classic case of integer division, so cast one as a float to
resolve the issue.

Change-Id: I485346e9ac0e38ea68e8e9cf620f9fc455f1cc7a
Signed-off-by: Roman Birg <romanbirg@gmail.com>
Merge "SystemUI: fix default navigation bar transparency value" into jb-mr2