Don't overly hide the Files preference.

This occurred this because the files preference was initialized during
displayPreference(), but the volume used to initialize it was not being
set until after the displayPreference() call. In this case, the files
preference would hide itself and never come back.

This fixes this by double-checking the Files visibility status whenever
we set the volume.

Change-Id: I0b1a7a9566e9caece39ec58706fbca034ef4c1c2
Fixes: 37790776
Test: Settings robotest