first_stage_mount: Create snapshot devices before launching first_stage_console

During device bringup, dynamic partitions may not be properly
configured by some sort of build or load misconfiguration. Diagnosing
such issues can be difficult without being able to see which partitions
are available and what they contain.

Aditionally, making logical partitions available to first stage console
permits early mounting of vendor partition and allows primitive
validation of vendor scripts without requiring full Android
environment. For instance, vendor_dlkm partition and modules can be
probed needing to have a full Android bootup.

Creation of logical partitions is done only when first_stage_console is
requested in order to have minimal impact on normal boot. Thus, only a
small refactor is required to split CreateLogicalPartitions out of
MountPartitions.

Bug: 174685384
Bug: 173732805
Change-Id: I82b7d77b9dc75af59b5e18b574e3eb99c8aff9e2
Signed-off-by: Elliot Berman <eberman@quicinc.com>