Suppress an erroneous lint message.

Contrary to lint message, we do need the appcompat-v7 library for Material
Design theming on pre-Lollipop devices.
Update XML namespaces for appcompat-v7.

Use the `http://schemas.android.com/apk/res-auto` namespace for the
`showAsAction` attribute in various XML files.
Improve theme.

Made text black rather than gray and added and improved theme colors. The
theme still needs some work, particularly the gray icons and light blue
accent color left over from Holo.