Support NDK r20 and Qt 5.14
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.
Reviewed By: apol
Subscribers: flherne, apol, kde-frameworks-devel, kde-buildsystem
Maniphest Tasks: T12520
Differential Revision: https://phabricator.kde.org/D26749