Collect telemetry data for System Server Loopers.

Adds LooperStats and LooperStatsService which set a static
Looper.Observer to record the count of dispatched messages and
cpu/latency information. The aggregated stats are collected by
Westworld as pulled atoms.

The collection is disabled by default and requires a manual call to
enable:

adb shell cmd looper_stats enable

Test: Unit tests and tested manually that the collected data seems
reasonable.

Change-Id: I7162fbca4e324f62887f95df3405205cbe4416ca