Make the default backup configuration "disabled; local transport selected"

By default out of the box, an Android build will have the backup mechanism in
its "disabled" state and pointed to the LocalTransport test transport.  We
do not want retail devices built without the Google backend to have backup
enabled out of the box; it would cause them to gradually grind away the
cache partition for no good reason.  On those devices with this change,
developers would need to enable backup manually (possibly using the normal
Settings UI; more probably using the 'bmgr' shell tool), but would no longer
also have to manually configure the active transport name.

Device vendors producing Google-enabled products will simply use resource
overlays to configure the default state and transport name for their builds.
When building a product that points to the Google backup transport by default,
the "def_backup_enabled" boolean resource should still be set to 'false' --
the Google backup disclosure activity supplied by GSF will take care of
enabling the backup services if the user opts in to it.  (Basically, vendors
will never have to overlay the def_backup_enabled resource -- the default
value of 'false' is correct for any retail device regardless of whether it
can use the Google backup transport.)

In the SDK build, the default transport will remain the local one, but
the default enable state overridden and set to "true".  This is the ideal
situation for developers: all aspects of the backup mechanism immediately
operative with no manual configuration needed.

Change-Id: I866f8f627b023b338bc7757e61604e6d8a901a34