fs_mgr: overlay backing, force filesystem for f2fs

Under the condition of an existing, but broken, filesystem on the
scratch device, force recreation. Add the -f flag for f2fs,
already present for ext4.

Test is enhanced.  The test needs to check, and if necessary wipe
out any existing overrides and reboot before running the remainder
of test.  Report which overrides are being used, and how they are
setup.  Fail if they are not as expected.  Improve efficiency of
checks by reducing the duplication of commands issued to the device.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: I9ac4fac952e334c00b089fec0b73b2cd2cc0af5c