Skip enabling dm-verity for live GSI when needed

Currently the dm-verity for live GSI is always enabled, even if the
disable bit in the top-level /vbmeta is set. We should skip setting up
dm-verity on live system.img when adb disable-verity is ever set.

Bug: 124291583
Test: adb disable-verity, then boot live GSI
Test: fastboot flash --disable-verification vbmeta vbmeta.img,
      then boot live GSI

Change-Id: Id52d20d0b2e56dfa7de8f866dcc989b82a96c879