[SP26.2] add a method in NetworkStats that removes empty entries

In current design, entries with zeros are preserved after
addition/subtraction. These entries are not very useful
and lead to difficulty of verifying the result of
addition/subtraction.

However, change the behavior in the original NetworkStats
is considered risky in current stage.

Thus, this change provide a function that could remove these
empty entries in tests.

Test: atest FrameworksNetTests
Bug: 152827872
Bug: 150644692
Change-Id: I40a76935d55712b8083ee1e17e137a8a4ef5e029