[HACK]fs_mgr: force recovery+wipe if data looks wiped

We do the recovery+wipe if
 - the mount fails and
 - the partition is forceencrypt
 - the partition seems wiped (== the 1st 4KB are all 0x00 or 0xFF).

This is a hack until we get the flashstation up and running with f2fs.
Bug: 15731906

The alternative would be for the user to manually enter
recovery and wipe his device.

Bug: 15747366
Change-Id: Ic8f3ef59f3d7d01d5d93d8e4c01502080f422157
Signed-off-by: JP Abgrall <jpa@google.com>