Get themed resources in KeyButtonView

When saving a customized button layout for the navigation bar, the
buttons load resources from the theme applied to systemui, regardless
if the navigation bar is being themed by a different package.  This
is not correct behavior.

Change-Id: I4e44ef517c2ea9ac3828e790f6a4b850726bc840
REF: NIGHTLIES-1377
(cherry picked from commit 526fa6d4)