fs_mgr: overlay check shared blocks for / if /system is not

For bringup devices that do not have right-sized partition, and if
/system is now root, check / to be sure.

Consider unshare blocks for ext4 as dead code, provide a strong
message to caller to provide all the dependencies to overlayfs.

Test: adb-remount-test.sh
Bug: 130327601
Change-Id: Iffa7c5f24d8f409e71f89fe9ece274d8c476f6fc
Merged-In: Iffa7c5f24d8f409e71f89fe9ece274d8c476f6fc