Upgrade to the latest gradle plugin, and use SDK Manager.

Jake Wharton's SDK Manager plugin for Gradle will manage setting up
up local.properties and keeping the various dependencies, sdks, etc
up to date. This should make it easier for people to build.

This fix also helps by making the Crashlytics plugin always enabled,
whereas before, we'd enable it manually for release builds and keep
it disabled for debug builds.
Favor using submenus over using two separate menus.