fs_mgr_overlayfs: Setup remount scratch dir with encoded mountpoint name

Instead of using the basename(mounpoint) as partition scratch dir name,
which could lead to name collision, use the normalized and encoded
mountpoint as scratch dir name.

Bug: 243503963
Bug: 306124139
Test: adb-remount-test
Change-Id: I5a64f17bc3b88f0ce42bd0c5779c8dd23a07917f