Fix NetworkStatsApiTest failed on Q device

Currently, the addEntry method is used in constructor of test,
which is not correct since there is no such method in Q devices.

Thus, initialize of NetworkStats variables outside of constructor.

Test: atest NetworkStatsApiTest
Test: atest CtsNetTestCasesLatestSdk:NetworkStatsApiTest on Q device
Bug: 150644692
Change-Id: Ibf2f8118c459a8d7a0992deca8f0f339ccd1bcea