init: mount_handler: system-as-root (legacy)

On marlin, dev.mnt.blk.root is empty.  Issue is shared for all
devices that are system-as-root.

/dev/root /proc/mounts entry exists before the associated block device
is instantiated by ueventd.  As a result when the device shows up the
root mount is updated late when the next mount inotify trigger occurs,
delay which we will accept.  But the property entries are added before
removed in the loop which causes the ultimate property entry for root
to report empty. Add /dev/block/dm-0, remove /dev/root, for property
dev.mnt.blk.root.

Fix is to change to Remove before Adding.  Remove /dev/root, then add
/dev/block/dm-0.

On system-as-root as well, can not just use fstab.  Determine if a
dm-verity reference is wrapped around system and use that instead.

Add some additional filtration of loop and APEX mounts to reduce
property noise.

Fix issue with creating the std:string line holder from getline(3).

Test: manual on marlin
Bug: 124072565
Change-Id: Ief2e1a6f559cbcbc87273fc2db35c675bb972f43