prevent splash image from moving on android (bug 971310)

https://bugzilla.mozilla.org/show_bug.cgi?id=971310

It stops moving.

Before https://www.dropbox.com/s/yat1b4yqel2t1p7/broken-spinner.mp4
After https://www.dropbox.com/s/3hzxi7gf5o50cgw/fixed-spinner.mp4