stack::gatt Use nodiscard for API to ensure return values handled am: 012cdfd2

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/3084765



Change-Id: Iaa3b8605f551d42f75e5ad6c31f5ec90f8eae11d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>