lightbox: Add "download" button in bottom app bar (Android implementation)

[chimnayajith]

This issue tracks the Android-specific implementation of the download button in the lightbox bottom app bar.

The original feature request (#42) covers both Android and iOS, but the implementations need to be handled separately.

References
Parent feature: #42

[nsamuelreddy]

Hi! I would like to work on the Android implementation of this feature.
Could you please assign me this issue if it’s available?
Thanks!