Move some MTRDevice_Concrete-specific constants to MTRDevice_Concrete. (#36152)

Author: bzbarsky-apple

src/darwin/Framework/CHIP/MTRDevice.mm

@@ -38,11 +38,6 @@
 
 #import "lib/core/CHIPError.h"
 
-// TODO Should these string definitions just move to MTRDevice_Concrete, since
-// that's the only place they are used?
-NSString * const MTRPreviousDataKey = @"previousData";
-NSString * const MTRDataVersionKey = @"dataVersion";
-
 @implementation MTRDeviceDelegateInfo
 - (instancetype)initWithDelegate:(id<MTRDeviceDelegate>)delegate queue:(dispatch_queue_t)queue interestedPathsForAttributes:(NSArray * _Nullable)interestedPathsForAttributes interestedPathsForEvents:(NSArray * _Nullable)interestedPathsForEvents
 {

src/darwin/Framework/CHIP/MTRDevice_Concrete.mm

@@ -58,6 +58,10 @@
 
 static NSString * const sLastInitialSubscribeLatencyKey = @"lastInitialSubscribeLatency";
 
+// Not static, because these are public API.
+NSString * const MTRPreviousDataKey = @"previousData";
+NSString * const MTRDataVersionKey = @"dataVersion";
+
 // allow readwrite access to superclass properties
 @interface MTRDevice_Concrete ()