Add xml pregrants for appfunction access

These pregrants, run on every fingerprint change, will allow a system
agent access to system targets. The pregrant files have the following
format:
<pregrants>
  <agent package="com.example.agent">
    <target package="com.example.target.1"/>
    <target package="com.example.target.2"/>
  </agent>
</pregrants>

Flag: android.permission.flags.app_function_access_service_enabled
Bug: 413087113
Test: atestAppFunctionAccessPregrantTest
Change-Id: I17fa3f7df39ed24db0602225629699ffdd98a25a