Allow whitelisted plugins on user builds

Plugins should only run on user builds if explicitly whitelisted.
It's also necessary to hold a signature permission:
  com.android.systemui.permission.PLUGIN

Test: atest PluginInstanceManagerTest
Test: atest PluginManagerTest
Test: manually try to install plugin on user build (whitelisted or not)
Bug: 111414690
Change-Id: If17b13f4caef677d641cba84b491b65c8135679b
diff --git a/packages/SystemUI/res/values/config.xml b/packages/SystemUI/res/values/config.xml
index 11bd392..6e7c720 100644
--- a/packages/SystemUI/res/values/config.xml
+++ b/packages/SystemUI/res/values/config.xml
@@ -498,4 +498,7 @@
 
     <!-- Allow dragging the PIP to a location to close it -->
     <bool name="config_pipEnableDismissDragToEdge">true</bool>
+
+    <!-- SystemUI Plugins that can be loaded on user builds. -->
+    <string-array name="config_pluginWhitelist" translatable="false" />
 </resources>