Configure backing device max_ratio for FUSE filesystems.

By default FUSE filesystems have a max_ratio of 1%, meaning only 1% of
dirty pages on the system can belong to a FUSE filesystem before we
start writing back pages (and throttling, if writeback can't keep up).
This limit is useful for untrusted filesystems, but in our case, we
trust the FUSE filesystem. Since FUSE writes result in writes to the
lower filesystem, FUSE should take at most 50%. Let's start with
changing max_ratio to 40%, to avoid needless throttling.

Bug: 159254170
Bug: 159770752
Test: inspect /sys/class/bdi manually after boot
Change-Id: I467e3770fc4afba0a08fa480c0b86aa054c8b875