[Thread] use immutable BorderRouterConfiguration am: ed24de9b64

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3210650

Change-Id: I01283f52c345bc04445bf9277b1d0af5c62c9990
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>