Themes: Full support for PA GlobalActions dialogs + ancient derp fix

- Use Theme Engine UiContext for creating dialogs.
- Add theming to reboot confirm dialog(s), send UiContext to ShutdownThread aswell.
- Fix dialog not being recreated when calling showDialog() and dialog being not null is nullified
  without being recreated (we don't call handleShow() there).
  Introduced here https://gerrit.paranoidandroid.co/#/c/128/ but not affecting the final result,
  now we have Theme Engine tho we need it.

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
Conflicts:
	policy/src/com/android/internal/policy/impl/GlobalActions.java
	services/java/com/android/server/wm/WindowManagerService.java