init: overlay: allow fs_mgr_overlayfs_required_devices to report partitions

init add support for picking up partitions if specified by
fs_mgr_overlayfs_required_devices() as /dev/block/by-name/<partition>

Test: adb-remount-test.sh
Bug: 119885423
Change-Id: I6a4c3d9b5c2b406178f0acf0a86c52ab17209537