UID mapping to provide app name and version.

The UID map is updated by StatsCompanionService, which listens to broadcast
updates indicating that an app was updated/installed or removed. Also,
the entire map is updated when statsd first connects to the companion
service. Also, there is a way for metrics producers to subscribe to
updates, so that they can know when an app was upgraded.

Test: Created new unit-test for mapping and manually tested for install
and remove. Did not manually test the app upgrade.

Change-Id: I6676ae5c93b75c72d9badabb36aa9c40006db07d