Create KeepaliveStatsTracker for Duration metrics.

Create a new class to store duration metrics of AutomaticOnOffKeepalives
for the DurationPerNumOfKeepalive proto.
This tracks the duration of time that the AutomaticOnOffKeepaliveTracker
contains a given number of concurrent keepalives and the duration of
time that it contains a given number of active concurrent keepalives.

Bug: 273451360
Test: atest FrameworksNetTests
Change-Id: If31ccbc7b02e6ad3ff1646e92e55ca7c8662e47c
3 files changed