Update Android toolchain files to reality

Summary:

• NDK r18 does not ship GCC anymore, it's only clang now
• same for STL, there's only libc++ there
• API level 14 is not supported either anymore, the minimum supported by the current NDK is 16
• The minimum supported API level by KF5 right now is 21 though, so use that as the default

Reviewers: apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D19016