Add file contexts for AdServices APEX

Test: Build
Bug: 220336612
Change-Id: Iab64d228a5edcd3a9f71b59c5adf3a9460cd1947
diff --git a/apex/com.android.adservices-file_contexts b/apex/com.android.adservices-file_contexts
new file mode 100644
index 0000000..9398505
--- /dev/null
+++ b/apex/com.android.adservices-file_contexts
@@ -0,0 +1 @@
+(/.*)?           u:object_r:system_file:s0