[SP18.2] Create TestableNetworkStatsProviderCbBinder

Currently, in OffloadController unit test, mock object is used
to verify callback sent from provider. However, verify a mock
method running in different thread, is not supported if there
is no additional thread synchronization mechanism applied.

This is fine now since all the callbacks fired from
OffloadControllerTest are on the same thread with caller.
But it is not necessary true in subsequent patches that fire
callbacks from handler thread.

Hence, introduce a testable provider callback that based on
a thread-safe ArrayTrackRecord to provide the functionality
to verify mock callback fired in a different thread.

Test: m -j
Bug: 149467454
Change-Id: I2edf887ccee5f205368a294f5f9990edb818332d