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.