Post UI update later a while to avoid screen sluggish as possible

When users turned on/off the saver mode, the system is busy in update
configurations and the dark/light theme, we should avoid updating the UI
in the same time to makes users feel the animation is not smooth

Bug: 201752975
Test: make -j64 RunSettingsRoboTests
Change-Id: I4c28af267616f796438a505e84ce376321c74981