init: Pass a uevent regen callback to libsnapshot.

In first-stage init, libsnapshot needs to know how to wait for device
paths, since ueventd isn't running yet. We do this by passing a callback
wrapping BlockDevInitializer.

Bug: 173476209
Test: device boots into first-stage init after full VABC ota
Change-Id: I9a87f98824e65a9379cb0b99c001e28cccd9d205