Switch to 'custom' when MediaSizeWidget updates

When the user changes the dimensions in the MediaSizeWidget make
sure the 'custom' option is invoked for validation and event
propogation.

Bug: 67684
Change-Id: I88c3f3dcc7667b3ffdd2a8a8433cb47b82ba1531
Merge "Switch to 'custom' when MediaSizeWidget updates"