Camera: Block shutter button during mode switch

Video recording will crash if start recording before startpreview is
done, so we need to disable shutter button before preview is ready.
Move the function which block shutter button after showVideoSnapshotUI
as the button will be enabled inside this function.

Change-Id: I9b7c4e352bd1f15742a4bdb7494a2e53f25d7a99
CRs-fixed: 545612