make tidy attribute tri-state

Bazel boolean attributes can only support true or false states, but the
clang-tidy logic changes for modules which don't set a specific value.
Therefore, the tidy attribute should really be a tri-state of "true",
"false", or "" (unset).

Test: b test //build/bazel/rules/...
Bug: 268681923
Change-Id: I85fbaa612d175f475757db806cea6c018901fe3e