Move list of SystemUI services to xml

Previously the services of SystemUI were defined in SystemUIApplication.

This patch moves the service list to res/values/config.xml. So that it
gets easier to customize services of SystemUI.

Bug: 63874929
Bug: 62602530
Test: Ran and confirmed SystemUI started without error

Change-Id: I7f0321c46e562288b2cad5201ae120c37e853908
diff --git a/packages/SystemUI/proguard.flags b/packages/SystemUI/proguard.flags
index 8cc2ce2..5b8881c 100644
--- a/packages/SystemUI/proguard.flags
+++ b/packages/SystemUI/proguard.flags
@@ -15,6 +15,7 @@
 -keep class com.android.systemui.statusbar.tv.TvStatusBar
 -keep class com.android.systemui.car.CarSystemUIFactory
 -keep class com.android.systemui.SystemUIFactory
+-keep class * extends com.android.systemui.SystemUI
 
 -keepclassmembers class ** {
     public void onBusEvent(**);