Fixe some stuff.

Addresses these bugs:

3061847 - With no headers, PreferenceActivity crashes
2888426 - minor typo in DevicePolicyManagerService.ActiveAdmin.writeToXml()
3159155 - IllegalStateException:"Can not perform this action after
    onSaveInstanceState" while dismissing a DialogFragment
3155995 - PopupWindow.showAtLocation does not respect LayoutParams

Also tweak the new fragment APIs to use abstract classes instead of
interfaces as base classes.

Change-Id: I9c0b4337fe0e304b737b5f7c2762762372bb3020