Refactoring of deleting stats in NetworkStats based on uids.

In follow-up commits, current API would create new NetworkStats
every time when 464xlatAdjustment wants to filtered out some
uids.

This commit refactors it to delete stats in-place to get better
performance.

Bug: 118602783
Test: atest FrameworksNetTests
Change-Id: I858f95d1fa7733111786243b4e261ce8a70a068d