The check box for Compress using ETC1 (split alpha channel) cannot be unchecked.

1. import a sprite into the editor
2. click on the Android settings tab in the Import Settings
3. click on the Override for Android box to check the box; the Compress using ETC1 (split alpha channel) box will appear checked
4. click on the checked Compress using ETC1 (split alpha channel) box

The box does not get unchecked.