mii/manager: Make use of unused lower bound in GetRandomValue()

Previously, the lower bound wasn't being used and zero was being used as
the lower bound every time this function was called.

This affects the outcome of some of the randomized entries a little bit,
for example, the lower-bound for beard and mustache flags was supposed
to be 1, not 0.

Aside from these cases, the bug didn't affect anything else.