Fix UIDevice support enabled for WatchOS 26 on Xcode 26

When building a WatchOS target with Xcode 26, the build fails due to a missing UIDevice call on the Watch SDK. It appears that in the initial WatchOS 26 betas that the check for `<UIKit/UIDevice.h>` in `RLMApp.mm` now passes for Watch targets when it shouldn't. This change adds a separate check for `TARGET_OS_WATCH` not being defined before defining the `REALM_UIDEVICE_AVAILABLE` flag.