Ignore empty frames in ime-controller.

Locking the screen was causing the ime insetssource to
report an empty frame. This isn't a valid frame for animation,
so don't use it in the animator.

Bug: 156287058
Test: enter split-screen, open ime in second split, lock/unlock.
Change-Id: Id99b744670b8be583389b763c3180d958e6777ae