Explain properly when apps can open multiple hosts' links

The app link settings "Open by default" detail view was showing
only one host under label "Supported links," improperly implying
that the app would only open that host's links.  The summary now
matches the 'App links' view's, explaining "Open $HOST and related
URLs" for apps with more than one.

Tapping on the item has always shown the full list of hosts
advertised by the app, and this remains unchanged.

Bug 22769535

Change-Id: I2be409df64639eb0f2b43a06e3195796c3a3df8b