The signing section was using $HOME/Android/Sdk instead of $ANDROID_SDK_ROOT, preventing from signing the APK if the Android SDK is not installed at the default location.
Currently, this just allows us to remove the donation button on Google
Play without using the deprecated installer APIs.
In the future, this should allow us to also release multiple versions of
Catima (for example: WearOS is a commonly requested feature, but this
needs non-free dependencies, which may not be okay to all users).
