handleBackspace should always send KEYCODE_DEL for InputType.TYPE_NULL

This is a fix only for the Bayo branch because this part in master has
been changed.

The corresponding fix in master is I295eeb5f9f0f1f07e919bf54122d003be150a174

bug: 11797053
Change-Id: I779be039ebf992de5d246c7a9d9509623fc3c120