[MS39.1] Remove the looper from NetworkStatsSubscriptionMonitor

OnSubscriptionsChangedListenerHandler(Looper looper) is
hidden, there is no way to pass a looper to it in the test.
This change partially revert aosp/1340040 to fix hidden
API dependency. And will use a different approach to address
this problem in later stages.

Test: atest NetworkStatsSubscriptionMonitorTest
Bug: 204830222
Bug: 213280079
Change-Id: I429be3578d599de9a5a3f78b02387c20744b37d3