Stop the created handler threads

The created handler thread will stay there for a while until
all the tests are finished. This is considered as a leakage
which will take the resource from the devices. In low end
devices, this may possibly impact the performance. The created
threads should be closed explicitly.

Test: atest FrameworksNetTests
Change-Id: I332d55d8effdb975fd9a8953b19910a7fb25e426