I already had Android Studio and the Android SDK installed. I later added Flutter and the Flutter SDK. Here is my problem: When I run
flutter doctor
It shows,

that I do not have the Android SDK installed.
You can configure the Android SDK for your project using
flutter config --android-sdk /path/to/android/sdk
The PATH variable also should contain the SDK, like
export PATH=$PATH:/usr/local/opt/android-sdk/emulator:/usr/local/opt/android-sdk/build-tools/26.0.1:/usr/local/opt/android-sdk/platform-tools:/usr/local/opt/android-sdk/tools
/usr/local/opt/android-sdk and 26.0.1 might not match your system. That's just an example.
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