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This change extends the original work to add a size limit over which
wifi is required to download a file.

First, this change adds a second size limit, over which wifi is
recommended but not required.  The user has the option to bypass this
limit.

Second, this change implements dialogs shown to the user when either
limit is exceeded.  These dialogs are shown by the background download
manager service when a download is started and found to be over the
limit (and wifi is not connected).

I'm including one small fix to the unit tests needed from the previous
change.

Change-Id: Ia0f0acaa7b0d00e98355925c3446c0472048df10