Merge NetworkStatsService handler changes.

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Remove the need of accessing handler in NSS unit test

Currently, to wait for handler becomes idle, specific message
is used and the test would wait for condition variable to be
open when the message is processed.

However, this is already done in the HandlerUtils. Thus,
there is no need to post such message manually in the handler.

Test: atest FrameworksNetTests
Bug: 150664039

Change-Id: Iab32b2dbab01634ca159dcb90fc9f929d1fed1a2
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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
=====

Bug: 150664039
Change-Id: If256ed4437ddcbcc72a6f766cff2f4cc512ee3f7
Merged-In: If256ed4437ddcbcc72a6f766cff2f4cc512ee3f7
(cherry picked from commit 2a6439d7)