Update keystore-statsd proxy for logging RkpPoolStats atom.

This CL accommodates some updates to RkpPoolStats AIDL atom introduced
during RkpPoolStats logging in keystore.

Ignore-AOSP-First: No mergepath to AOSP.
Bug: 184301651
Test: Statsd test drive script.
Change-Id: Id412b34f0726ffc8dda32afbf95c1e8b2318935f