Add carrier config keys to control the data cycle.

Adds the following keys: KEY_MONTHLY_DATA_CYCLE_DAY,
KEY_DATA_WARNING_THRESHOLD, and KEY_DATA_LIMIT_THRESHOLD.

This allows carrier apps to control the monthly data cycle reset date,
as well as fill in data warnings and limits based on plan details.

Bug: b/34385985
Test: No explicit tests for the API additions, but will add tests to the
follow up review which adds functionality that depends on these fields.
Change-Id: I2aee5cdbc4b707e28062830f1143c694ccae13cd