Fix issues #4087362 and #4087356

4087362: Provide a safer way to call DialogFragment.dismiss()
4087356: PreferenceActivity.invalidateHeaders() can cause
IllegalStateException: Can not perform this action after onSaveInstanceState

These are very safe; the first is just a new public API that
allows you to use an existing feature in DialogFragment, and the
second just uses the version of commit that avoids the failure if
happening at a point where the operation would be lost if restored
from the last state (which is no big deal for preferences).

Change-Id: I53971c9fb1efdcd599694cdcd4585b81afc156b8