BlackTheme: Set overlay category as system palette

- We apply this overlay as system_palette in the
  ThemeOverlayController as it changes the system colors.
- Also overlay package should have the right category but
  we missed to update so disabling this overlay via
  ThemeOverlayController was broken.

Test:
This overlay is getting disabled after turning off "Pure black" option in the settings.
- adb shell cmd overlay list

Change-Id: Ibd8a32b457c9f5a2ce2ddf1c52e0ed7ee65ef5d0
diff --git a/customizations/BlackTheme/AndroidManifest.xml b/customizations/BlackTheme/AndroidManifest.xml
index b0cef48..2a169f2 100644
--- a/customizations/BlackTheme/AndroidManifest.xml
+++ b/customizations/BlackTheme/AndroidManifest.xml
@@ -8,7 +8,7 @@
         android:versionCode="1"
         android:versionName="1.0">
     <overlay android:targetPackage="android"
-        android:category="android.theme.customization.theme_style"
+        android:category="android.theme.customization.system_palette"
         android:priority="1"/>
     <application
         android:label="@string/android_overlay_black"