Give users and devices control over sdcardfs.

Instead of relying only on kernel support for sdcardfs, give each
device the ability to quickly toggle between sdcardfs and FUSE.  Also
add the ability to users to explicitly enable/disable the behavior
for testing and debugging purposes.

Bug: 27991427
Change-Id: Ia57f12f6ead888ab85101004bdd8c8003537b116