Integrate `MediaPicker` with WordPress Android

[wzieba]

This can blow up to more detailed tasks when we'll recognise bottlenecks/API change requests.

[renanferrari]

This task is being currently being tackled as a trial project here: wordpress-mobile/WordPress-Android#15608

Update: this task is paused for now (more details here: pbMoDN-35Z-p2). Hopefully, we'll be able to get back to it later on.