lightbox: Animate entrance/exit of the lightbox's top and bottom app bars

[chrisbobbe]

Currently the lightbox's top and bottom app bars respond to touch by appearing and disappearing. Currently they just abruptly appear and disappear, but they should nice entrance/exit animations. The React Native app has them slide in and out; we could do that or perhaps a fading animation.

[chrisbobbe]

Mentioning here, since it came up in #224 (review):

• This needs to work on devices where the OS intrudes on the app's space at the top and bottom of the screen (like the iPhone "notch" for the camera etc.).
• The layout should look reasonable even when the user changes their preferred text size in their device's accessibility settings.