PacketResponder: stop should wait for thread to join am: d3f5b7328a

Original change: https://android-review.googlesource.com/c/platform/frameworks/libs/net/+/2168406

Change-Id: Id2fed765d907df481815e7ada133e03ba7c7d3a5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>