Use mockito.after() instead of Thread.sleep

Because the change moves the call to hander, the test needs to wait
for a timeout to make sure the call is called. Instead of using the
Thread.sleep(), it's better to use mockito.after().

Bug: 29385425
Test: atest PackageMonitorCallbackHelperTest
Change-Id: Id6ac8d6cdbe28f31bae0837414c6970f1df6f608