I keep getting a notification saying "Choose input method" on Android

This is a feature of the Android operating system. The notification is visible whenever you are in a text field and you have more than one keyboard enabled (including the stock keyboard).

Tapping the ‘Input Select’ icon (represented by a keyboard) will also display this notification.

It is possible to hide this notification completely by hiding all the other keyboards installed on your device.

To achieve this:

1. Locate your device's Keyboard and input methods settings. These can normally be found within your 'Languages & input' menu.
2. Select 'Virtual keyboard'
3. Tap 'Manage keyboards'
4. Toggle all other keyboards to the 'Off' position

You'll need to remember to reverse the previous steps to make all your keyboards appear again.
 

Troubleshooting

If you can't see the 'Input Select' keyboard icon, you might have to drag the Android Notification Bar downwards to make your selection instead.