remount: prefer 'cache' backing storage for non-A/B devices

In non-A/B configurations system partitions often
have layout with not so much free space left, while having
large /cache partition.

In a dynamic partitions configuration 'remount' for backing
storage will user either:

-- /data partition, which is not guaranteed to have
enough space due to applications disk usage;

-- or super partition, which is tied to system images size
having a little room for growing.

At the same time, /cache is guaranteed to be free,
so non-A/B platform can force it to be used as
backing storage.

Test: remount
Signed-off-by: Oleg Lyovin <ovlevin@sberdevices.ru>
Change-Id: I68e621b884b2fe21a5c464b3deaf679186232eb3