Implement notifySystemPropertiesChanged

This is a Test API that notifies of updates to the system properties
when requested, received by callers of
SystemProperties#addChangeCallback

Test: atest android.view.inputmethod.cts.InputMethodStatsTest; check
logs for ImeTracker onProgress entries during text execution only

Change-Id: I8b81194524de822eab7538aab68675a677b5fd6d