clatd: move stop_loop() into main.c am: 8ab7e13332 am: 2c2543395c

Original change: https://android-review.googlesource.com/c/platform/external/android-clat/+/1574002

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I4f7f88e5bddef5508a2a28981cd352a04e2320e6