charger: add 400dpi symlink

* used by google bonito
* https://android.googlesource.com/device/google/bonito/+/refs/tags/android-9.0.0_r39/device-bonito.mk#18

Change-Id: Idd0b4640ee4896c86b25a5457fe889c28e49a283
diff --git a/charger/images/400dpi b/charger/images/400dpi
new file mode 120000
index 0000000..17ef829
--- /dev/null
+++ b/charger/images/400dpi
@@ -0,0 +1 @@
+xxhdpi
\ No newline at end of file