init: Make sure /dev/block/by-name/userdata is created in snapshot mode.

When userdata is used to store COW devices, we need to ensure we've
generated uevents for it.

This patch also refactors FirstStageMount to pass required devices
through arguments rather than a member variable.

Bug: 140761481
Test: manual test
Change-Id: Ie5e1c9699f084da5467a758eea41c7907fecc5ca