Update install app icon colors to spec:

The + in the icon is onSurface.
Everything else within the circle is material_color_surface_container_lowest.

spec: https://docs.google.com/presentation/d/1_f2nAcmZMywF0D1-9fJv1slUQVerIpJ0b1uUmZDpKXo/edit?resourcekey=0-nxjnkIp8z-ECp8angeQJZg#slide=id.g239ecda44ed_0_18

bug: 340307977
Test:
before: https://screenshot.googleplex.com/Bss6VHDt7JfjsEU
after: https://screenshot.googleplex.com/5qxqQ6segow2Nxh
Flag: None
Change-Id: I300e8513d15e428338cb1e20f65fbe3a0b62bc35
diff --git a/res/drawable/private_space_install_app_icon.xml b/res/drawable/private_space_install_app_icon.xml
index 4c167ba..cfec2b1 100644
--- a/res/drawable/private_space_install_app_icon.xml
+++ b/res/drawable/private_space_install_app_icon.xml
@@ -23,9 +23,9 @@
             android:pathData="M30 0H30A30 30 0 0 1 60 30V30A30 30 0 0 1 30 60H30A30 30 0 0 1 0 30V30A30 30 0 0 1 30 0Z" />
         <path
             android:pathData="M30 0H30A30 30 0 0 1 60 30V30A30 30 0 0 1 30 60H30A30 30 0 0 1 0 30V30A30 30 0 0 1 30 0Z"
-            android:fillColor="@color/material_color_surface_bright" />
+            android:fillColor="@color/material_color_surface_container_lowest" />
         <path
             android:pathData="M29 31h-6v-2h6v-6h2v6h6v2h-6v6h-2v-6Z"
-            android:fillColor="@color/material_color_on_surface_variant" />
+            android:fillColor="@color/material_color_on_surface" />
     </group>
 </vector>