[SP23] Address misc API review comment about NetworkStats

This change add several javadoc for NetworkStats and its Entry
class. Also remove IFACE_ALL constant accordlingly.

Test: atest FrameworkNetTests
Test: m doc-comment-check-docs
Bug: 148895143

Change-Id: Id1b8f41be79f847ad73197ff0e18258b6ee8a730
Merged-In: Id1b8f41be79f847ad73197ff0e18258b6ee8a730
(cherry picked from aosp/1256351)