Data Usage summary creates a dialog to change data warning level

Clicking the Data usage summary preference will open the same dialog
that billing cycle does to modify data warning level. The Dialog
was abstracted to be used outside of the Billing Cycle settings
fragment.

Test: Robo tests in next cherry-pick.

Change-Id: I70398d4656b6da4233b2bfa0da6ffa7402ead69b
Fixes: 26932938
(cherry picked from commit 9fad289c)