ANDROID: android-verity: Fix broken parameter handling.

android-verity documentation states that the target expectets
the key, followed by the backing device on the commandline as follows

  "dm=system none ro,0 1 android-verity <public-key-id> <backing-partition>"

However, the code actually expects the backing device as the first
parameter. Fix that.

Bug: 72722987

Change-Id: Ibd56c0220f6003bdfb95aa2d611f787e75a65c97
Signed-off-by: Sandeep Patil <sspatil@google.com>