Notify the keepalive is stopped after the slot has been released

Currently, the callbacks of stopping were fired when stop procedure
is started, because the upper layer apps only care about the reason
of stopping instead of stopping result. Thus, there is no need to
wait for the result comes back. However, this behavior generates
races if apps want to re-start keepalive immediately since the
resources are not released yet.

Fix: 134891441
Fix: 140305589
Test: atest com.android.server.ConnectivityServiceTest#testPacketKeepalives \
      --rerun-until-failure 1000

Change-Id: I987776a9211a50e964c4675b747bc10e203750f1