Gallery2: add loading progress dialog for FilterShowActivity

Add a progress dialog to limit access to dual cam filters until after
DDM is successfully loaded. Dual camera filter category will be
hidden by default until DDM is loaded and verified.

Conflicts:
        res/layout/filtershow_activity.xml
        res/layout/filtershow_main_panel.xml
        src/com/android/gallery3d/filtershow/category/MainPanel.java

CRs-Fixed: 929338
Change-Id: I9602c3fb31da37856b00b029d4e6c8d1e8ae3d71