Fix -Wswitch-enum errors in FBAMRestorableDevice.m

Summary:
Latest stables are blocked https://www.internalfb.com/shipyard/stable/fbobjc_stable/overview?session_id=1254665563091619 as a result of the enablement of We enabling [-Wswitch-enum for all Apple targets](https://fb.workplace.com/groups/mobile.sheriffs/permalink/29958097777145452/)

Add explicit handling for `AMRestorableDeviceStateUnknown` in two switch statements in `FBAMRestorableDevice.m` to resolve compilation errors.

The compiler with `-Werror,-Wswitch-enum` requires all enumeration values to be explicitly handled in switch statements. The `AMRestorableDeviceStateUnknown` case was missing from:
1. The `state` method at line 57
2. The `targetStateForDeviceState:` method at line 133

Both now explicitly handle the unknown state by returning `FBiOSTargetStateUnknown`.

Differential Revision:
D86477448

Privacy Context Container: L1246528

fbshipit-source-id: dbf49ad192db331e532352f439b4631a3a83d1b1

Author: Edward Lim

FBDeviceControl/Management/FBAMRestorableDevice.m

@@ -63,6 +63,8 @@ - (FBiOSTargetState)state
       return FBiOSTargetStateRestoreOS;
     case AMRestorableDeviceStateBootedOS:
       return FBiOSTargetStateBooted;
+    case AMRestorableDeviceStateUnknown:
+      return FBiOSTargetStateUnknown;
     default:
       return FBiOSTargetStateUnknown;
   }
@@ -139,6 +141,8 @@ + (FBiOSTargetState)targetStateForDeviceState:(AMRestorableDeviceState)state
       return FBiOSTargetStateRestoreOS;
     case AMRestorableDeviceStateBootedOS:
       return FBiOSTargetStateBooted;
+    case AMRestorableDeviceStateUnknown:
+      return FBiOSTargetStateUnknown;
     default:
       return FBiOSTargetStateUnknown;
   }