prop_context: correctly label all property_context files

split property context file in vendor and sytem were left untouched by
the recent changes. This was working accidentally because they were
still accessible to all domains as 'system_file'.

Bug: 36002573
Test: Boot sailfish to observe no new denials.
Test: 'adb sideload' OTA on sailfish successfully

Change-Id: I5bec058b59db83d2a431e9f7e91c5a09af7d2942
Signed-off-by: Sandeep Patil <sspatil@google.com>