Change overlay setup logic to be in line with remount logic

Overlays stopped working on some devices because they assumed system
would dedupe blocks. But since we are moving to new logic which simply
uses overlays for all dynamic/logical partitions, adjust (and simplify)
code to use that assumption.

Bug: 419320970
Test: adb remount works
Change-Id: I90bf2b2deee6729185a01be7ac843b4bf12fea5f