Hygiene: Introduce EchoOverlay initialisation class

Not as scary as it looks. The change to _buildOverlay simply changes
indenting, and cleans up to reflect new parameter. Tests still pass ;-)

Kill noticationLimit concept
Only expose what's necessary on mw.echo.overlay

Change-Id: I2671a41af8188c14ad4c910396afa0d9000b6051
Merge "Hygiene: Introduce EchoOverlay initialisation class"