Copy dummy userdata.img to sdk system images

Avd manager in Android Studio requires userdata.img in the downloaded
system image. As we no longer inlcude the real userdata.img, we copy the
dummy userdata.img to pass the check.
This cl does not impact real devices.

BUG: 110492064

Change-Id: Ia8377ce72e32c3556a5a27100085a07f43d9917c
Signed-off-by: Weilun Du <wdu@google.com>
diff --git a/build/sdk.atree b/build/sdk.atree
index 421f494..410d354 100644
--- a/build/sdk.atree
+++ b/build/sdk.atree
@@ -237,6 +237,7 @@
 system-qemu.img                                 system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/system.img
 vendor-qemu.img                                 system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/vendor.img
 ramdisk.img                                system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/ramdisk.img
+device/generic/goldfish/data/etc/userdata.img    system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/userdata.img
 data/misc                                        system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/data/misc
 system/build.prop                          system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/build.prop