Merge lp://qastaging/~albaguirre/mir/fix-1570327 into lp://qastaging/mir

Fix ApplicationNotRespondingDetection.failure_to_pong_is_noticed when running on a heavily loaded machine (LP: #1570327)

On a heavily loaded machine, the client session will fail to pong naturally, before the test disables responding to pings,
and before the test has a chance to register an observer.

ANR Observers should be notified of all currently unresponsive sessions when registering with the ANR detector.

Before this fix, you could replicate by running:

stress -c 100
../tools/detect_fd_leaks.bash "/usr/bin/valgrind" "--error-exitcode=1" "--trace-children=yes" "--leak-check=full" "--show-leak-kinds=definite" "--errors-for-leak-kinds=definite" "--track-fds=yes" "--num-callers=128" "--suppressions=<mir>/tools/valgrind_suppressions_generic" "--suppressions=<mir>/tools/valgrind_suppressions_glibc_2.23" "--suppressions=<mir>/tools/valgrind_suppressions_libhybris" bin/mir_acceptance_tests --gtest_filter=ApplicationNotRespondingDetection.failure_to_pong_is_noticed --gtest_repeat=1000