Don't do private app-dir permissions/quota on public volumes.

Don't do private app-dir permissions/quota on public volumes.

While looking at some emulator logs, I noticed that we fail to create
dirs like /Android/data/com.foo/cache on public volumes, because we try
to chmod it; public volumes go completely through FUSE, even for
Android/, and so these operations will fail, because the underlying
UID/GID is not setup correctly.

Really the only thing we really have to do on public volumes is create
the dirs, like we used to do.

Bug: 152618535
Test: manually verify cache dirs can be created successfully
Change-Id: I66e5d0873f1198123787943b17b468eadf0a853d