Register OverlayPlugins in StatusBar directly.

This removes the last call to getComponent(StatusBar.class).
Prior to this change, we would register a listener for OverlayPlugins
inside the SystemUIApp. Because they are directly related to the
StatusBar, it is safe and appropriate to have the StatusBar register
with the PluginManager itself.

Bug: 143224715
Test: atest SystemUITests
Change-Id: I5ecdf6a4dae05ba674bcbcd940b9f2655d2b1cbb