Add tests for device config changes

Test device config changes are propagated correctly to the watchdog.

* Keep polling until the config value becomes the expected one.
* Remove listeners at the end of each test - too many device config
  listeners will delay the notification significantly ( > 1 min)
  according to my local experiments.

Bug: 181820350
Test: atest PackageWatchdogTest
Change-Id: Ia5e89b69a8052f49e6400f6b22313249d523786c