Support TrafficStatsRateLimitCache

This is used in follow-up changes to rate-limit TrafficStats
get* API calls.

Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.TrafficStatsRateLimitCacheTest
Bug: N/A
Change-Id: I91bc71dda3a1b7c0b5525278b3c04d12492972fa