Merge "Add javadoc to NetworkStats related classes." am: e51a1d0254 am: daa3f0519a am: 7c457bff70 am: a7d3cbaa43

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

Change-Id: Ifcd393b558059c2d153dc2a419b3fe7617004830