Fix mounting /data on Virtual A/B devices without metadata encryption.

When using partitions backed by /data, for example during a Virtual A/B
merge or "adb remount" on a Virtual A/B device, the userdata block
device is seen as in-use when /data mounts in second-stage init. This
subsequently prevents mount() or e2fsck from working. Metadata-encrypted
devices are not affected, because dm-default-key provides a wrapping
block device that can be used exclusively.

This patch addresses the problem by detecting when userdata has
device-mapper dependencies. If it does, and the device is not
metadata-encrypted, we introduce a dm-linear wrapper around userdata.
It is created on demand, but like logical partitions, it exists until
the device reboots.

Bug: 134949511
Test: adb remount, cuttlefish boots
Change-Id: Ifbfea1591a6e58978fdaffd6ef889afabd10e270