init: refactor first stage to not require fstab

In order to support dm-linear devices, we need an additional first-stage
step to ensure that required devices are created. This must happen before
setting up dm-verity or mounting any first-stage partitions.

This patch refactors FirstStageMount so that having a compatible fstab
is optional. This will let us use InitRequiredDevices on systems that
would not otherwise perform first-stage mounts.

Bug: 78914864
Test: non-AVB devices still boot
Change-Id: I11265375a9900d983da8cabcc77d32c503ded02e