Make getTag() protected

There doesn't seem to be a way for a custom Tree implementation to provide its own tag-generating logic. `Tree.getTag()` is package-visible so can't be overridden by an external Tree. The result is that the tag is always `null` unless each logging call is preceded by `Timber.tag()`.

Would it be a good (or bad) idea to make `Tree.getTag()` protected?

Start by locating the Tree.getTag() entry point and review how custom Tree implementations and Timber.tag() are used. Confirm the intended visibility change preserves automatic tag generation for external Trees, then run the relevant existing tests for Tree tagging.

Written by the indexing model from the issue text.