Add get and set methods; deprecate data

[joews]

Setting the value through MaterialDateTimePicker#data(newValue) doesn't set the value correctly, and overloading data for get/set is not a friendly API design.

Bug: picker.set(newDatetime) does not set the picker's time where newDatetime differs from the picker's current time in both date and time.

• Add get, which always returns the current value as a moment
• Add a deprecation warning to data() / data(nextValue). Mark for removal in a future release.
• Fix behaviour of set where both the date and time change