fs_mgr: overlayfs: export fs_mgr_overlayfs_candidate_list

Refactor fs_mgr_candidate_list into fs_mgr_overlayfs_candidate_list
that reports all the possible candidates.  The caller is responsible
for filtering out any that have verity enabled.

Sundry improvements to the adb-remount-test.sh script to improve
stability and feedback.

Test: adb-remount-test.sh
Bug: 122602260
Change-Id: I2399f83d8ed77d8f3d2ad1405d0c187ccbace764