Execute all callbacks on the MainThread.

This is a precursory step which will allow us to delete the double
handlers that currently exist in WifiTracker.

Bug: 37674366
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java

Change-Id: Ieea1af712b80ea6ede358d7a1004e489e11c6009