mms: Fix FC when viewing slideshow

Add SlideshowPresenter to proguard.flags so that it can be found with introspection.
Fixes FC when clicking on received MMS slideshow and when trying to create new slideshow.

Change-Id: I4d34b39db5dc6e07da59587d051648e46ddc6803
JIRA: CYAN-5766
(cherry picked from commit 1bbdb2ada5d2babf49355d592a4df893eac12b4e)