Fix Preview to handle no items in preview state

On some devices, PickerViewModel might be destroyed to optimize memory
onStop(). Since all the selected items data is saved in PickerViewModel,
we may run into a situation where PreviewFragment is launched with no
selected items. We now handle this situation by going back to
Photo/Album grid from where PreviewFragment was launched initially.

Bug: 207844970
Test: Manual. Videos attached in the bug.
Change-Id: I7faf7d4ba31e04ec9fa6aebb29f60a92bdb9ddc2
1 file changed