Copy dummy userdata.img to sdk_addon 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
Test: Manual
1, Build with command "make -j110 PRODUCT-sdk_gphone_x86-sdk_addon".
2, Check if the dummy userdata.img appears in sdk addon image zip.
Change-Id: Icb88aac42921502288efe7355b8cd76704d5eb30
Signed-off-by: Weilun Du <wdu@google.com>