Handle renamed "super" device correctly

In the non-retrofit case, the metadata source partition will be
hardcoded as "super", but such a partition may not exist or may have
been overridden by super_partition=<something>. Make this work reliably
by plumbing the resolved metadata block device through, and using it any
time a partition called "super" is requested.

This also fixes a case in the first_stage_mount code that assumed that
detected a non-A/B, non-retrofit case by assuming the metadata source
partition would be called "super".

Bug: 113175337
Test: boot test on cuttlefish
Change-Id: I8ce1defb902e257eeb7170d697b378eba31faee3