Have NetworkStats implements iterable

Make NetworkStats be iterable and expose iterator as system API
which is better than expose size() and getValues().
This API could be used by the caller who needs to get entry
from NetworkStats.

Bug: 210073043
CTS-Coverage-Bug: 215061403
Test: build, TH
Change-Id: I6cb4c8f63d7067133f2722b2be7e16b4098a697d