Improve grid preview performance

After breaking down the time we spend in rendering preview under a different grid setting, I found out that we spend a huge amount of time loading the workspace. It takes a long time in non preview case (launcher workspace init), so to optimize we should try to cut down things that are not necessary for preview. Widget model loading (widget and shortcut updates) takes half of the time, and can be optimized with minimal risks / code changes.

Bug: 160662425
Test: Manual

Change-Id: I89029d0ddf6e2517077a0ba3fbbcfdcd60b268d9