Add getParent() on Preference.

Preference class now keeps track of its PreferenceGroup parent which
makes it easier to remove it from the preference hierarchy.

This required a new API method.

Test: Separate CTS CL being submitted along this one, see the topic.
BUG: 30599046
Change-Id: Ia9f274ef0315cb09a8c558e781b10f57efd28406