Make fuseblk use vfat context

* VFAT label allows us to use fuse like before and at the same moment
  doesn't allow some core domains to access labeled files/filesystem

* VFAT is a generic term for external storage, as seen in AOSP
  that now supports both VFAT and EXFAT under the generic sdcard_type
  label

* Many OEMs already follow this logic and thus this also allow to merge
  the genfscon rules if matching the ones already provided in vendor
  allowing some treble enabled devices to boot ( Eg: OnePlus 6 )
  ( see Change-Id: I2f3beb1a7922df1bf60fca09896878f5f7812281 )

Change-Id: I74f7a0d8c60f540381d7b3f2c30a9d7b496a509d