SettingsProvider: turn off mobile data roaming by default

Commit 23a9f51c moved the roaming data property from
a system property to xml config, but inadvertantly switched the default from false
to true.  This fixes it by making the default false again.

Change-Id: I25b7666a5c0b638992e8203621daca6c860c91b2
(cherry picked from commit 0365da6c)