recovery: Fix mounting /system with dynamic partitions.

When using dynamic partitions, the blk_device field in fstab_rec must be
translated to a /dev/block/dm-N node with
fs_mgr_update_logical_partition. However, init will not have created
these nodes to begin with since CreateLogicalPartitions is not called in
recovery. This patch addresses both issues.

Note that flashing system through fastbootd will not work while /system is
mounted.

Bug: 118634720
Test: manual test
Change-Id: I06c83309d09eab6b65245b1ed10c51d05398f23e