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
diff --git a/fastboot/device/variables.h b/fastboot/device/variables.h
index aa4d9fc..3b2d484 100644
--- a/fastboot/device/variables.h
+++ b/fastboot/device/variables.h
@@ -54,6 +54,8 @@
                            std::string* message);
 bool GetIsUserspace(FastbootDevice* device, const std::vector<std::string>& args,
                     std::string* message);
+bool GetIsForceDebuggable(FastbootDevice* device, const std::vector<std::string>& args,
+                          std::string* message);
 bool GetHardwareRevision(FastbootDevice* device, const std::vector<std::string>& args,
                          std::string* message);
 bool GetVariant(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);