fs_mgr: set "partition.system.verified" when AVB is used in a A/B device

In a A/B device, system partition is mounted by kernel as root.
In vboot 1.0, the dm device name of system partition is "system" with
the following configuration in kernel command line:
    - dm="system none ro,0 1 android-verity /dev/sda34"

In AVB, the dm device name is switched to vroot as:
    - dm="1 vroot none ro 1,0 5201456 verity 1 ..."

When sending ioctl DM_TABLE_STATUS to query status, we should use "vroot" as the
dm device name for AVB. But still pass "system" for the callback function to set
property [partition.system.verified] instead of [partition.vroot.verified].

Bug: 36900078
Test: Use AVB to mount system in a A/B device, checks the property exists
      [partition.system.verified]
Test: Use vboot 1.0 to mount system in a A/B device, checks the property exists
      [partition.system.verified]
Test: Checks 'adb remount' will output warning message:
        - dm_verity is enabled on the system and vendor partitions.
        - Use "adb disable-verity" to disable verity.

Change-Id: Iaee7eb2b00b03729bc07fa24f1b449488716d2ea