* Update Android NDK to version r19c
* Update Android API versions to:
* API 24 on arm and aarch64 targets
* API 28 on x86 and x86_64 targets
* Unified headers were removed in NDK 16
* Refactor the NDK and SDK installation scripts
* OpenJDK version 8 must be kept: it appears that
the Android tools do not work with more modern OpenJDK
versions.