Create aconfig flag for enabling SDK system feature cache

This cache avoids binder lookups for SDK defined system features by
seeding processes with a very compact lookup for known system feature
types.

This complements compile-time codegen for fixed system feature versions
defined via build flags; as only a handful of such system features
are explicitly fixed at build time, and not all devices opt in,
the cache remains valuable for the longer tail of features and devices.

The flag is currently unused, but will be used in
ApplicationPackageManager to conditionally use this new cache.

Flag: android.content.pm.cache_sdk_system_features
Bug: 375000483
Test: m
Change-Id: Ib7592328b674d311710c9f1e44e61c704dc3a2b4