setSelectedHeader() when switching to fragment

Without this change, passing a EXTRA_SHOW_FRAGMENT value
for a two panel preferences activity will cause the correct
fragment to be shown. However, it will not cause the
correct header to be highlighted.

Bug: 15675746
Bug: 5045558
Change-Id: I18d8e15dd104e8f39ae1ccbb11ab17fd1bf7462e