Summary:
We currently rely on the Paper UIManager calling `uiManagerWillPerformMounting` to flush the animated operations queue, which includes starting and stopping animations. This mostly works right now because Fabric always starts after Paper, but sometimes Paper doesn't fire `uiManagerWillPerformMounting` for a while, which can delay an animation starting.
To fix this, I force a flush of the queues on the UIThread whenever start or stop is called. This should be safe because the order of animation operations is still preserved, and start/stop are (almost?) always called in dedicated event handler loops, so any other updates like changing the way nodes are attached should already have been processed from a previous JS execution loop.
Reviewed By: JoshuaGross
Differential Revision: D14313502
fbshipit-source-id: 2a2b0c614fd1a591bd04b6b3fafcc09ff6c9d6e7
3e40837a85