Release keepalive slot after stopped

Currntly, keepalive slot is released when stop() is called. Next
starting keepalive can use the same slot number while previous
keepalive is still stopping. When the previous keepalive is
stopped, the incoming as will be processed by the new keepalive.

This change release keepalive slot after the result of stopping
has returned. Thus, newly created keepalive cannot allocate the
same slot number while lower layer is still processing stop event.

This change also disable flaky assertions that are caused by
test port has been occupied by other process.

Bug: 129512753
Test: 1. atest com.android.server.ConnectivityServiceTest \
         #testNattSocketKeepalives --generate-new-metrics 100
      2. atest FrameworksNetTests --generate-new-metrics 10
      3. simulate the fail case manually.

Change-Id: I1991627545519ee5cb408a3df3a006f710f4af7b