Add a simple SystemFeaturesCache abstraction

This cache supports a dense representation of all SDK-defined feature
versions. Used together with build-time defined system features, it
allows efficient runtime feature checks for the P95 set of queries while
minimizing any per-process memory impact (<1KB per cache, ~7x faster
than IPC cache).

A follow-up CL will integrate this into PackageManager.hasSystemFeature
queries behind a flag.

Bug: 203143243
Test: atest FrameworksCoreTests:android.content.pm.SystemFeaturesCache
Test: atest CorePerfTests:android.content.pm.SystemFeaturesPerfTest
Flag: android.content.pm.cache_sdk_system_features
Change-Id: I58e9f9cf322a9d074ba773342f5c18a47c9f066b