Merge "Add javadoc to NetworkStats related classes." am: b69c56683c

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1917019

Change-Id: Ic92ff264664e714cdcf0a8deadf0e9a254ce752e
tree: dff774d31c1ed7d683e33cccc3a33c292936688f
  1. core/
  2. framework/
  3. framework-t/
  4. service-t/
  5. services/
  6. tests/