Android 5.0: Back does not close the open menu #265

On Android 5.0 (emulator, API 21) Back does nothing while the menu is open: the PopupWindow keeps the focus and the menu closes only with OK or Esc. The release build 2.19.1 does the same, so this does not come from #263; found while checking PR #264.

Likely cause: below API 23 a PopupWindow without a background drawable does not wrap its content in the view that handles Back. Fix to try: set a transparent background drawable on the menu popups, and check Back on the API 21 emulator.

Newer versions are not affected as far as checked; #30 relies on the focusable popup taking Back.