Move the app to /system_ext

As mentioned in go/partitions-in-r, we plan to forbid hidden API in
product partition.
So this app should be moved to /system_ext because it uses hidden API.
It is not permanent, so they can be moved back to /product due to
needs if there isn't hidden usage anymore.

Bug: 137908189
Test: m
Test: check whether device boots successfully
Change-Id: If15f854a33aef9098c1ff27bad2a3b49742b633b
diff --git a/Android.mk b/Android.mk
index 9135970..eb83a9f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -31,7 +31,7 @@
 
 LOCAL_PACKAGE_NAME := ThemePicker
 LOCAL_PRIVILEGED_MODULE := true
-LOCAL_PRODUCT_MODULE := true
+LOCAL_SYSTEM_EXT_MODULE := true
 LOCAL_OVERRIDES_PACKAGES := WallpaperPicker2
 
 ifneq (,$(wildcard frameworks/base))