fastbootd: Add getvar is-force-debuggable

Export ro.force.debuggable property so that the user can check image
compatibility prior to running fastboot flash.
For example, if is-force-debuggable is yes, then the "system" and
"vendor" build fingerprint must match.

Bug: 191649082
Test: fastboot getvar is-force-debuggable
Change-Id: I772d98253f58ba208d5803e18b589ff693deebd0