Register pullers in separate thread

This cl modifies getStatsService to use the blocking getService, but
also makes the binder call in a separate thread to not block the client
thread. This is needed because it is possible for pullers to be
registered before statsd starts, and calling checkService before statsd
is up will fail. We also would never receive the binderDied to register
it, because we would never have a binder object to linkToDeath on.

Bug: 147682855
Test: atest LibStatsPullTests
Change-Id: I68c04bc24c7fe066eca88cab4f6a76885581c1ee