Fix Viewer slide load on backend change - take 2

Split the slide switching logic (including load/unload) into
setCurrentSlide(), while keeping setupCurrentSlide() to deal with config
only.

Change-Id: I5bd2363ffd401c1b756217f845d4dbd16d6be5d6
Reviewed-on: https://skia-review.googlesource.com/94864
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed