build: Android cross-compilation support for `Qt 5.x`

## Summary

- Android build support was removed due to Qt 5.15’s incompatibility with NDK r25 and the migration to C++20 (see [[build, test, doc: Temporarily remove Android-related stuff #30049](https://github.com/...)](https://github.com/...) and [[ci: Android NDK has too old libc++ #29360](https://github.com/...)](https://github.com/...)).

- [[build, test, doc: Temporarily remove Android-related stuff #30049](https://github.com/...)](https://github.com/...) and [[test: #29007 follow ups #29636](https://github.com/...)](https://github.com/...) note that Android support will be reinstated after migrating to CMake and upgrading Qt to 6.x.

Start by reading the linked issues #30049, #29360, and #29636 to understand the removed Android build support and its stated prerequisites. Then inspect the repository's current build configuration; done means Android cross-compilation support is restored following the CMake migration and Qt upgrade described in the issue.

Written by the indexing model from the issue text.