ipacm: fix the VTS error

On VTS IPACM_HAL unit-test, since wlan
tethering devive won't up, therefore the
addDownsream events are all cached in
IPACM_HAL and keep accumulated until
reaching IPACM's max cached and seeing
test-case starts failing. The fix is
to clean up the cache on Stopoffload.

Bug: 65612227

Change-Id: I24f410f5c5ae3a6cad0e14293f266b067f643389