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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 '7c8405513675c47205a68c0644f6046eebefc5a3':
  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.