Bug 48995: Add acceptable use 'tutorial' to app

Initial port of the acceptable use tutorial from iOS app. This version uses static images and is not yet animated. Indicator circles on the pager are done using Apache-licensed https://github.com/JakeWharton/Android-ViewPagerIndicator -- imported via maven. Currently the tutorial launches when we reach the login screen (as when first installing the app or creating a new account from system settings). You can either 'back' out of it or page through and hit the 'yes' button.

GitHub: https://github.com/wikimedia/apps-android-commons/pull/20
Change-Id: Ibc444102e28a55bfa7bbae601ff0c56268a0c7dd