Remove Handler abuse from OngoingTransition

[CherryPerry]

    private val checkFinishedRunnable = object : Runnable {
        override fun run() {
            if (transitionElements.any { it.isPending() }) {
                handler.post(this)
            } else {
                finish()
            }
        }
    }

This is 100% CPU load on the main thread while transition animations are running, making them less smooth.

[zsoltk]

@SamuPS FYI