Address review followup comments for NetworkStats

This change includes:
1. Mark groupedBy*() deprecated and throw after android U.
2. Modify map() to mapKeysNotNull().
3. rename clearInterfaces to withoutInterfaces and refactoring.
4. Modify tests accordingly.

Test: atest FrameworksNetTests:android.net.connectivity.android.net.NetworkStatsTest
Fix: 296149902
Fix: 296150018
Change-Id: I5d97422ba957a212c0c5fbc1eee3f8b174343348
3 files changed