Implement new perfd<->statsd ShellSubscriber comm.

Because we no longer linkToDeath against a binder object to detect if
the cmd process has died, we detect deaths by checking if writes fail.
ag/10476582 proves that writes fail if the cmd process dies.

Test: m statsd
Test: bit statsd_test:ShellSubscriberTest.testPushedSubscription
Test: bit statsd_test:ShellSubscriberTest.testPulledSubscription
Bug: 150619687
Change-Id: I44a777ffff11e5b9298912b2906063c65e9009eb