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am bbb2b8d4: am 2abe99b9: am b4e1c857: am 25d193e9: am 7c840551: add Nav Drawer lesson and revise other lessons with updated API recommendations for back stacking and a revised structure that separates itself from the design-centric terminology.

* commit 'bbb2b8d4474ac5de157fed37f09f1955559fc8e4':
  add Nav Drawer lesson and revise other lessons with updated API recommendations for back stacking and a revised structure that separates itself from the design-centric terminology.