Don't send PLUGIN_CHANGED broadcast unless something changed

If a plugin changes the layout (e.g., CameraCutout changing status bar
height), the preference screen will call persistBoolean, which would
always cause PLUGIN_CHANGED to be broadcasted. This causes bad infinite
loops.

Test: install CameraCutout plugin and navigate to Settings > Sysui tuner
> Plugins and watch it loop a lot

Change-Id: Id340bbfb0864508f1895bbd1e6cb2849a839bad8