Fix concurrent modification exception in KeepaliveTracker

In aosp/951200, the clean up function delete the item in the
hash map that holds the record while iterating it, where the
list used to iterate the records is backed by the hash map,
so changes to the map are reflected in the list and caused
the concurrent modification exception.

Bug: 132341736
Test: 1. atest com.android.server.ConnectivityServiceTest \
         #testNattSocketKeepalives --generate-new-metrics 300
      2. atest FrameworksNetTests --generate-new-metrics 10

(Clean cherry-pick of aosp/959599)

Change-Id: I9cdfe6f6d11c5400c856cc30a33ff4a44ba9d811
Merged-In: I0481a469ee23231e5f0ab738a06b5e09f6cdb680