Support NDK r20 and Qt 5.14

Summary:
This changes from using the toolchain file provided by CMake to using the
one provided by the NDK, as even recent CMake can't build successfully
with r20. However this is a rather invasive change, the interface and
variable names differ.

The Qt 5.14 changes are less risky, as most of this is parallel to the
support for older versions.

Test Plan: Local builds with 5.14/r20, 5.14/r18 work, the Docker SDK isn't tested yet, and there's some remaining issues with 5.13 and older NDKs I don't fully understand yet. The resulting apks with 5.14 install, and work for QQC2 content, but fail to start Kirigami apps.

Reviewers: apol

Reviewed By: apol

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

Tags: Frameworks, Build System

Maniphest Tasks: T12520

Differential Revision: https://phabricator.kde.org/D26749