Make some NetworkStats APIs system-current

Move iterator and NetworkStats.Entry getters to system-current
according to API council feedback. This reverts parts of
ag/17117903.

Ignore-AOSP-First: Parent CLs does not exist in aosp yet.
Test: TH
Fix: 225168182
Change-Id: Ia7fdf8d31a96a26b0bf1682f462292b051560477
3 files changed