Use the dm target for userdata

Framework or applications can try to find userdata dm target from "/proc/mounts"
which gives "/dev/block/mapper/userdata".

That causes breaking the assumption in all the places.

Bug: 311165039
Change-Id: I9ea1b4589cdd52021d9807f7240c2e4b6d6d05ef
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>