fastboot: Fallback to "raw" partition type if fastboot hal isn't present

Fastboot format fails to wipe any partition that doesn't at least return
"raw" partition type. Also both android.hardware.fastboot@1.0-impl.pixel
and android.hardware.fastboot@1.1-impl-mock return FileSystemType::RAW
so I assume this is fine.

Change-Id: I5707bddb1ba32edb6359858853d7b1afbf138b9f