Settle on var names ANDROID_SDK_ROOT/ANDROID_NDK_ROOT

Summary:
Motivated by https://mail.kde.org/pipermail/kde-android/2016-April/000048.html

Would update wiki page, once committed.

Reviewers: rahn, nienhueser

Reviewed By: nienhueser

Projects: #marble, #android

Differential Revision: https://phabricator.kde.org/D1474