flow: Add 'LayoutEvent' type to 'ComposeBox'
flow: Add 'LayoutEvent' type to 'ComposeBox'

Import `LayoutEvent` from the semi-obscure place it is defined
and use it to type the layout event handler.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile