Gallery2: Fix the superposition of two images issue

When we zoom out to film mode from single photo mode with more than two
fingers, PhotoView will receive multi onScaleBegin/onScaleEnd streams.
But when mModeChanged is true, it will only notify PositionController to
beginScale but not to endScale, then mInScale remains in true state. So
when we back to single photo mode from film mode, focuse photo can't do
scale animation in Box.startSnapback().

According to the logic in onScaleEnd(), we will ignore other scale begin
notification if film mode has been changed.

CRs-Fixed: 551984

Change-Id: I468d932adab0a3540f107501db4d0ea4b7ffbb10
Merge "Gallery2: Fix the superposition of two images issue" into du44