Introduce Scribe gesture: InsertModeGesture

Introduce a new gesture called InsertMode that inserts space in the
editor for user to be able to comfortably write in an area.

Bug: 260141015
Bug: 254727073

Test: atest InputConnectionEndToEndTest

Change-Id: I53be7f1a4476fd0cb7b621779462f77634a65691