Add tests for custom canned_fs_config.

There are no tests to verify the generated action for the canned fs
config entries, so add some.

Also update the prop's docstring to reflect the actual logic.

Bug: 275209284
Fixes: 275280970
Test: soong tests

Change-Id: I37f2a8640bf4c307068a77db7a635c9bbeb9f38f
diff --git a/apex/apex.go b/apex/apex.go
index 1f700c6..2977f35 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -103,12 +103,13 @@
 	// to avoid mistakes. When set as true, no force-labelling.
 	Use_file_contexts_as_is *bool
 
-	// Path to the canned fs config file for customizing file's uid/gid/mod/capabilities. The
-	// format is /<path_or_glob> <uid> <gid> <mode> [capabilities=0x<cap>], where path_or_glob is a
-	// path or glob pattern for a file or set of files, uid/gid are numerial values of user ID
-	// and group ID, mode is octal value for the file mode, and cap is hexadecimal value for the
-	// capability. If this property is not set, or a file is missing in the file, default config
-	// is used.
+	// Path to the canned fs config file for customizing file's
+	// uid/gid/mod/capabilities. The content of this file is appended to the
+	// default config, so that the custom entries are preferred. The format is
+	// /<path_or_glob> <uid> <gid> <mode> [capabilities=0x<cap>], where
+	// path_or_glob is a path or glob pattern for a file or set of files,
+	// uid/gid are numerial values of user ID and group ID, mode is octal value
+	// for the file mode, and cap is hexadecimal value for the capability.
 	Canned_fs_config *string `android:"path"`
 
 	ApexNativeDependencies