Add LRU cache with expiry class

This is a no-op refactoring which extracts an
LruCacheWithExpiry<K, V> superclass from
TrafficStatsRateLimitCache to provide an LRU
cache with an adjustable expiry duration and generic types,
allowing entries to be automatically removed from the cache
after a certain duration.

This is needed for follow-up changes to provide similar
functionality with other types without manually handling expiry.

Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.TrafficStatsRateLimitCacheTest
Bug: 343260158
Change-Id: Ia413b8c6e73c376dd821cd2bb676ef996cd85a91
2 files changed