Custom picker: Allow selecting images in different folders

[nicolas-raoul]

Goal: the user selects some pictures from a folder, then navigates back to the folders list, then goes to another folder, selects some more pictures, and performs actions on all selected pictures as a whole: upload, or mark as not for upload.

Scenario: I have a set of 10 pictures to upload but only one requires a privacy blurring and thus was saved in the blurring app's folder. Thus I select 9 pictures in my Camera folder and 1 in my blurring app's folder, then upload the 10 as a set (with the same depictions/categories).

[rohit9625]

I'll consider this issue while migrating custom selector into jetpack compose.