[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
2 files changed