fs_mgr: overlay: use alternate for backing storage

When we have multiple physical super partitions, it is prudent to
take the alternate super physical partition in its raw entirety
rather than to construct a logical partition out of the current
super partition's limited remaining space.

SideEffect: alternate boot partitions destroyed for the sake of debug
Test: adb-remount-test.sh (including manually disabled to check
      both code paths).
Bug: 119885423
Change-Id: I368d3a5619f69de3b0d1fcad73c4b42d25d0d969