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

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

Change-Id: I470002e1e5455ef36660d0590b98a3de148c0c21