first_stage_mount: Move CreateLogicalPartitions to DoFirstStageMount

In preparation for later commit:
   first_stage_mount: Create snapshot devices before launching
first_stage_console

Bug: 173732805
Bug: 174685384
Change-Id: I6b77690c7cf68f6235c99bf4ff897b0ee41c4d0e
Signed-off-by: Elliot Berman <eberman@quicinc.com>