fs_mgr: Use ro.boot.veritymode

If verity state is managed by bootloader, it will pass the verity
mode to the kernel in the androidboot.veritymode command line
parameter. Init copies the value to the ro.boot.veritymode property.

Check for ro.boot.veritymode in fs_mgr and use the value to set
dm-verity mode. If this property is not set, store verity state in
metadata as before, if a storage location is specified in fstab.

Bug: 21605676
Change-Id: Ife3c978c133248432c302583d3b70e179605fe42
(cherry picked from commit ac5c1224)