Add SystemFeaturesMetadata.maybeGetSdkFeatureIndex

Add a simple index lookup method for feature names. This returns:
  * [0, SDK_FEATURE_INDEX) for PackageManager-defined system features
  * -1 otherwise

This will be used in a follow-up to efficiently compute a dense array
of cached system feature versions that can be used for fast lookup in
client processes.

Bug: 375000483
Test: atest SystemFeaturesMetadataProcessorTest
Flag: EXEMPT currently unused
Change-Id: I145c08f7eb58b832534f62950357a18f7b4e3651