buildozer: add print_all_lists command for '+' list expressions (#1525)

Buildozer's mutating list commands (`add`, `remove`, `replace`,
`substitute`) and `print_comment <attr> <value>` traverse `+`
concatenation expressions via `edit.AllLists`, operating on all
top-level `*build.ListExpr` operands in expressions such as `["a", "b"]
+ select(...)` or `DEPS_VAR + ["a"]`.

However, `print <attr>` relies on `Rule.AttrStrings` (`build.Strings`),
which only matches a single `*build.ListExpr` and falls back to
returning the raw formatted expression string whenever `+` is used.
Unconditionally changing `print <attr>` would break existing callers
that distinguish pure list literals from expressions containing
variables or `select(...)`.

Add `edit.AllListsStrings(e build.Expr) []string` to extract string
literals across all top-level `*build.ListExpr` operands returned by
`edit.AllLists(e)`, and expose a `print_all_lists <attribute(s)>`
command in Buildozer that prints the combined string elements from all
top-level list literals as a list (e.g. `["a"] + select(...)` prints
`[a]`, and `VAR + ["a"] + ["b"]` prints `[a b]`).