Ignore the asynchronous result while stopping keepalive

Currently, onStopped callback are synchronizely triggered when
stop() was called, since the framework don't really care
about the result of stopping keepalive.

However, if keepalive failed to stop for some reason, the
handleStopKeepalive was called mistakenly and trigger additional
callback that fail the test case.

This commit is the behavior change prior to state machine
refactoring, and introduce a stopping state for ignoring the
result in the stopping state.

Bug: 129512753
Bug: 123988249
Test: 1. atest com.android.server.ConnectivityServiceTest \
         #testNattSocketKeepalives --generate-new-metrics 100
      2. atest FrameworksNetTests

Change-Id: I4fa94e0740ba488fb5fe7ac7c3812c195dd0ec4c