We don't want to support only 'relative' validity, so don't assume that the
Validity property will always be a uint32 value.
Instead, we define the Validity propery as '(uv)' tuple, where the first value
(a MMSmsValidityType) specifies the type of validity, and the second value is
a variant formatted accordingly to what the validity type specifies (e.g. a
uint32 value if the type is MM_SMS_VALIDITY_TYPE_RELATIVE).
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