set-verity-state: Use ro.boot.veritymode to determine current verity state

On a device that don't use overlayfs remount (e.g. no EXT4 dup blocks;
this can be simulated by patching fs_mgr_wants_overlayfs()), if we run
disable-verity or enable-verity twice in a row then the second
invocation would not suggest a reboot:

  adb disable-verity
  > Successfully disabled verity
  > Reboot to take effect...
  adb disable-verity
  > Verity is already disabled
              ^^^ this is WRONG! verity is disabled only after a reboot

It behaves like this because it suggest a reboot only if the vbmeta
verity (HASHTREE) flag is changed.
Read the ro.boot.veritymode property instead to determine the current
dm-verity state and suggest a reboot by comparing current and future
verity state:
* If AVB verification is disabled, then ro.boot.veritymode is undefined
  (probably empty), don't suggest reboot in this case as it's pointless.
* Otherwise suggest a reboot if the new state (which would take effect
  after reboot) differs from the current verity state.
* Reference:
  https://android.googlesource.com/platform/external/avb/+/master/README.md#handling-dm_verity-errors

Bug: 241688845
Test: adb-remount-test
Test: Run "adb enable-verity" & "adb disable-verity" multiple times
Change-Id: If1df5bee6e5dcbda580b3dff6c32da93d08bbb46