libfiemap: Disable loop mapping code.

On devices without metadata encryption, we use loop devices rather than
device-mapper + dm-linear + FIEMAP. Devices without metadata encryption
should not exist, since libfiemap was introduced with Android R, which
requires metadata encryption.

It is possible to retrofit an Android Q device with Virtual A/B, which
is what Pixel 4 did. However those devices can only upgrade to
Android T, and they had metadata encryption anyway.

If there are any Android Q devices that retrofitted Virtual A/B in R,
didn't have metadata encryption, and need to upgrade all the way to V,
then we can recommend they make WrapUserdataIfNeeded() unconditional.

Bug: N/A
Test: fiemap_image_test, vts_libsnapshot_test
Change-Id: I7be0507527b967166676c8b136b8758f5e69ba6b