Remove setHandler in NetworkStatsService

Currently, internal handler is set by setHandler after
constructing NSS object. This was introduced in ag/866187 to
access the handler in the unit test.

However, the design put NSS in a bad situation where all classes
that need handler or executor could not be final and need to be
dynamically allocated in order to get a valid handler.

Thus, since the usage of handler is removed in previous patch,
this change eliminate setHandler by initializing the handler in
the constructor.

Test: atest FrameworksNetTests
Bug: 150664039

Change-Id: I794a24d00b0ca9fdc78091e7b9ab7307e0f034b7