Now the class initially uses smaller timeout, and expand it when detecting
multiple changes in a short period.
This CL makes the UI look more responsive especially on the message list +
message view mode. e.g. Starring on the message view will quickly be
reflected to the message list.
Bug 3024799
Bug 3027832
Change-Id: Ie2d44c3769d43e3fd0f54ee526556eb3bad5e288