Clear default browser setting when a new browser app is installed

Even if you have explicitly named a default browser app, when you
add a new browser app to the system it now assumes that perhaps you
might want that to be the new default, so it puts you into the
explicit ambiguity state.

Also fix a bug in the factory-default-browser handling; it was not
examining the set of available browsers.

Bug 22120412

Change-Id: I183ed87a51be1e4aa65457f05b98e406bb7b495d