Upgrade to ubuntu bionic and Qt 5.12

Summary:
Qt 5.6 is too old for KF5 these days.

Qt >= 5.9 no longer provides a binary installer for Android, so we're now compiling Qt ourselves (qtbase+qtdeclarative+qtlocation, for now).

Qt 5.12 requires the use of clang and a more recent NDK than 10e, so I upgraded to 15b (14b worked too).
Can't go up all the way to 18b because that requires a more recent cmake than the one in ubuntu bionic...

I reordered sections in the docker file mostly to skip doing the apt-get every time I change a Qt-related or NDK-related env var change, but also because unpacking the NDK now requires "unzip" so we need to install that first.

Using clang to compile KF5 seems optional, but if we want it for consistency, then this needs -DCMAKE_ANDROID_NDK_TOOLCHAIN_VERSION=clang in scratch/cordlandwehr/kdesrc-conf-android.git. I'm wondering why that is separate btw, the setup is related to this docker, so why don't we put the kdesrc-buildrc here and use Docker's COPY command?

Test Plan:
The docker container gets built fine, all the way to KArchive.

KCoreAddons doesn't build though, for multiple reasons which seem more related to its code, to ECM, to kdesrc-conf-android.git... IOW nothing the dockerfile itself can fix, unless we realize we need a different NDK version after all.

Reviewers: cordlandwehr, scarlettclark, bcooksley, apol

Reviewed By: cordlandwehr

Subscribers: vatra, apol

Tags: Android

Differential Revision: https://phabricator.kde.org/D12864