system_server: assert app data files never opened directly

Add a compile time assertion that app data files are never
directly opened by system_server. Instead, system_server always
expects files to be passed via file descriptors.

This neverallow rule will help prevent accidental regressions and
allow us to perform other security tightening, for example
bug 7208882 - Make an application's home directory 700

Bug: 7208882
Change-Id: I49c725982c4af0b8c76601b2a5a82a5c96df025d