Notify upper layer when trying to stop non-existing keepalive

Currently, KeepaliveTracker would expect event from network agent
for start/stop result. However, there is no event when failed to
find a valid keepalive. Thus, KeepaliveTracker would unable to
release slots if previously failed to start keepalive on mobile
data.

Bug: 135510870
Test: 1. atest FrameworksTelephonyTests
      2. atest android.net.cts.ConnectivityManagerTest \
         #testSocketKeepaliveLimitTelephony

Change-Id: I76a0cc117fb7a9071137de73ee5bb5494a397b9b