Add a dark theme

Summary:
BUG: 375376

This revision adds dark mode support to the app ( T7044 ). It does so by injecting
theme information into each activity, and making sure that all Views
define their colors by reference to theme attributes.

In order to make this work, all of the buttons with images (like the list
of available devices) now are tinted according to the theme.

While all versions of android support the theme, only devices running
Android ICS or higher will have a toggle button in the drawer.

Test Plan: Open all the screens, both with and without the dark theme on.

Reviewers: #kde_connect, mtijink, #vdg, nicolasfella

Reviewed By: #kde_connect, mtijink, nicolasfella

Subscribers: apol, ngraham, nicolasfella, mtijink

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D11694