Introduce MetadataSyncAdapter.

This class is used to index runtime metadata in sync with the static
metadata for app functions.

Flag: android.app.appfunctions.flags.enable_app_function_manager
Test: atest FrameworksAppFunctionsTests -c
Bug: 357551503
Change-Id: Ifd031cced3bb974c5e7c183f402e6e3798078025