Remove NetworkStats hidden API dependencies from StatsPullAtomService

This patch removes the following dependencies,
- Reimplement removeEmptyEntries.
- Remove NetworkStats.Entry() constructor.
- Remove size() and use hasNext() to check if the stats contains any
  entry.

Bug: 210073043
Test: BytesTransferredTest
Change-Id: Ib2a4b38e4412cbeb6b18e5ed50bc6a7c1ca79b00