Snapshot persist.sys.fuse into sys.fuse_snapshot

This will allow us to receive values from server flags and store them
in persist.sys.fuse without risking flag consistency during a boot.

Test: manual - flip persist.sys.fuse both ways and make sure FuseDaemon
works as expected.
Bug: 140803239

Change-Id: I4424770926f98bccda06dab6a447ee5637e5eb6e