Fix transitions between single and multi-pane.

Transitions in and out of multi-window were causing navigation history
losses + incorrect views to be displayed. The history loss was caused by
the fact that single-pane mode was using Activities but multi-pane mode
was using Fragments. Translating history between those two would be
challenging. Because of that Fragment approach was also applied to
single-pane mode. That required some changes in back press handling and
initial logic when creating the PreferenceActivity.

Test: Separate CTS CL being submitted along this one, see the topic.
Bug: 32201932
Change-Id: I5e6a6a21c34e813d1c3522ae7f5e33f43887c602