Ensure public methods are running on handler thread

Store the handler that should be running all the public methods and
use it by comparing its thread with the current thread.

Also refactor the tests to use and run on a test handler thread.

Bug: 273451360
Test: atest FrameworksNetTests
Change-Id: I053f1043a3b518728c4b30b94eec1d33fbfe39c0