Adds client API for interacting with statsd.

This API will primarily be used by GmsCore to send updated configs.
Also, sending a config will implicitly notify the StatsD that this
client wants to know when it should request data for this config.

We send a broadcast so that all interested subscribers can know if
data needs to be pulled.

Test: Manually tested that sending broadcast works via new adb
command added in StatsService.

Change-Id: I23cdd1df706036e14b32c3d01af30c3d4af819fa