Add getter for the fields of NetworkStats.Entry

NetworkStats is going to be moved into mainline module and
there're some external callers have dependencies on it.
To replace those hidden APIs with formal APIs, NetworkStats
needs to expose some getter methods to eliminate the dependencies
on hidden API.

Bug: 210073043
Bug: 204830222
CTS-Coverage-Bug: 214881138
Test: FrameworksNetTests
Change-Id: I85bb6b27bd17bad16c9693d95b569d0ec5fbb785