remount: Refactor fs_mgr_overlayfs_teardown

This pulls code for mapping the scratch device into a separate function.
It also avoids implicitly failing by passing an empty device string.
Finally, it calls GetScratchDevice, to remove a caller of the deprecated
method fs_mgr_overlayfs_scratch_device().

Bug: 134949511
Test: adb remount and adb_remount_test.sh
Change-Id: If4a543d3fa26af3f8578ec8b236859c8e4d9bfd8