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