Start snapuserd if needed as soon as possible during second-stage init.

snapuserd is used as a user-space block device implementation during
Virtual A/B Compression-enabled updates. It has to be started in
first-stage init, so that updated partitions can be mounted.

Once init reaches second-stage, and sepolicy is loaded, we want to
re-launch snapuserd at the correct privilege level. We accomplish this
by rebuilding the device-mapper tables of each block device, which
allows us to re-bind the kernel driver to a new instance of snapuserd.
After this, the old daemon can be shut down.

Ideally this transition happens as soon as possible, before any .rc
scripts are run. This minimizes the amount of time the original
snapuserd is running, as well as any ambiguity about which instance of
snapuserd is the correct one.

The original daemon is sent a SIGTERM signal once the transition is
complete. The pid is stored in an environment variable to make this
possible (these details are implemented in libsnapshot).

Bug: 168259959
Test: manual test
Change-Id: Ife9518e502ce02f11ec54e7f3e6adc6f04d94133