build.gradle: Use the latest build tools version.
Add a small version of the launcher icon for the Amazon App Store.
Activity tests: Set the theme to avoid an exception from AppCompat.

Avoid this exception when using ActivityUnitTestCase:
java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
Move the feature graphic into a sub-directory
Slightly improve the feature graphic and add a promo graphic.
README.md: Remove the text about it not being official.