Rework make_partitions logic when preserve_ephemeral is set

This patch reworks the logic around make_partitions when rebuilding an
instance with preserve_ephemeral set to true. The reworked logic will
no longer wipe the current partitions when rebuilding an instance with
preserve_ephemeral set.

Partial-Bug: 1317647
Change-Id: I15fc0d6323698eaa4c30b8aaa7fb740f51c035bc
Merge "Rework make_partitions logic when preserve_ephemeral is set"