Post runnables instead of sending messages.

Gaining access to a Handler's Looper can be problematic in custom
Handler implementations. Instead of creating a new internal Handler
to dispatch the method calls on we simply post Runnables to the
given Handler.

Test: adb shell am instrument -e class android.net.NetworkRecommendationProviderTest -w com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Bug: 34845471
Change-Id: I6fe380a92aebd630781abe5853f94737bcfeaa5f