Fix runtime crash on KitKat and prior.

This is a follow up CL to the previous CL [1], in which we started
calling Window#setNavigationBarColor(int) when the window visibility is
changed.

One thing we missed is that calling Window#setNavigationBarColor(int) on
KitKant or prior devices would result in a runtime crash.  Hence with
this CL we do not call that method unless the OS version is N or leter,
because specifying Color.TRANSPARENT would make sense on N+ devices.

  [1]: I14d9490e00caa852035a05830e76114cbe6af8f2
       6c04339c

Bug: 22564251
Bug: 27302540
Change-Id: Ib7299dd8c3dad4271f8fac453e690c83bda4a954