So how did Tonto get out of jail?

He didn't answer. He just fed the bird. The kid tells him to stop because it wasn't alive. There was a crow present at the massacre. I think we're led to believe Native American mysticism had a hand.

Upon hearing the question, the old Tonto snooped around (from his spot in the display) so my take is the younger Tonto also snooped around and somehow got a moment to get himself out of the prison cell.

Tonto could escape from shackles and a chain made of Bethlehem steel when the Lone Ranger declared it hopeless. Tonto is resourceful.