Brush directory .meta files part 1

I was thorough when cleaning out the previous bad export and its
follow-on fixes, so these .meta files got lost. Bring them back
from Poly Toolkit, which is the first unitypackage to have directories
for the new doublesided brushes.

It might not really matter whether they change or not, but it's easy
enough to keep them unchanged, so I did.

Change-Id: I66ff4d134d154784e551a0e9933614423fc04d13