fs_mgr: overlay: test assumption wrong if verity is enabled

Test script assumes overlayfs takeup after disable-verity, although
that can happen if verity is already disabled, it is not the case
for devices that just had it disabled.  Move takeup and first stage
init tests downwards to where the condition is guaranteed.

Differentiate the test for devices that support or have verity, and
those that do not.

Fortify test to also use /data mount to check init first stage limit.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: I5241d6c97bf374614e50aa0aa18a1b9d118be093