Add drop shadow to toolbar items in AppDetails.

This is so that feature images with white backgrounds to not completely obscure the icons.

Unfortunately Vector Drawables are not able to support dropshadows. It seems the easiest approach is to take existing icons, add a drop shadow, then render as .png files of various sizes.

I used the Android Studio "New -> Image Asset" and selected the ic_back, ic_share, and ic_more icons. I can't figure out why they made the icons bigger then the ones used by the SDK. If that is particularly incorrect, I'm happy to resize the icons down before merging.

The icon sizes themselves in the drawable-* dirs are 96px, 64px, and 32px as this is what Android Studio created after selecting "New -> Image Asset".

Also, I'm not actually sure if the drop shadow is pronounced enough. Thoughts?

Fixes #1097 (closed).

Below you can see master at the top, with small, non-drop shaddowed icons. Under that is this branch: