⬆️ Migration to AndroidX & Updated Butterknife version

[AlejandroHCruz]

• It compiles and runs successfully.
• In the CheckoutActivity, had to manually point to the R.id.snackbar_text instead of the id from the legacy support library.
• The new Butterknife requires the @BindView & @OnClick annotations to use R2 as mentioned in their repo instructions: https://github.com/JakeWharton/butterknife
• Had to manually replace import android.support.annotation.NonNull with import androidx.annotation.NonNull.