Add a mechanism for products to specify default active app linkages

We now have a new tag type in the existing SystemConfig XML files:

    <app-link package="..." />

Packages mentioned here are treated as though the user had instructed
the system to always open the app rather than a browser when touching
an HTTP/HTTPs URL that is handled by the app.  This default state can
of course still be turned down by the user if they choose.

Bug 21595872

Change-Id: I2824064a891bba3867ee86f81d609d22e83aef7a