Fix ConnectivityServiceTest#testNetworkCallbackMaximum

Although commit 893a762c fixed some flakyness issues in
testNetworkCallbackMaximum so that it became stable when ran on its own,
it introduced a new source of random failures because instead of
registering callbacks after callbacks until a limit was reached, commit
893a762c changed the test logic to push the assertions right up to
the theoretical limit.

More precisely when registering and unregistering PendingIntents in a
loop, not introducing some delay for checking that previous
PendingIntents have been effectively unregistered can cause the test to
fail. This patch fixes this issue.

Bug: 32561414
Bug: 62918393
Test: runtest frameworks-net
      testNetworkCallbackMaximum now succeeds 100 in a row on sailfish

Change-Id: I086817a738ab99fd53ba76ca8faada6151f46472