bug(android): backspace from start of field in Google Keep does not join to previous field

This is an instance of a more general issue with backspace. Noted on my Samsung phone with Keyman 7.0.303-beta.

At the start of a Google Keep checkbox field, you can normally press backspace to join that field to the previous one (i.e. deleting the 'line break'). However, with Keyman for Android, pressing backspace at the start of the field has no effect. We should match the behaviour of other keyboard apps and Keyman on other platforms, where backspace at start of context is passed through to the app for its own consumption.