Use a framework resource to name a factory-default browser app

If there is no resource-named default but there is a single factory-
installed browser app, that app is made the titular default.

This also introduces a permission guard on attempts to set the
default browser or the app-link state.  These operations are now
contingent on the existing SET_PREFERRED_APPLICATIONS permission.

Bug 21778406

Change-Id: Id099bb9c4141f28917546492657cd2fba472e6b6