Refactor buildKeepaliveMetrics to return DailyKeepaliveInfoReported

This also updates the tests and prepares them to assert the other fields
of DailyKeepaliveInfoReported. Currently, only DurationPerNumOfKeepalive
is asserted.

Bug: 273451360
Test: atest FrameworksNetTests
Change-Id: I727849fa8a08d66fe276909b353a75861c23d775
2 files changed