Fix right-bottom corner edge to curve in PS container.

This happens when there isn't enough apps to fill in the private space container.

* Added test to check the correct regions are being set depending on the apps in the list.
* Need to set ROUND_BOTTOM_RIGHT to the last app item and the decorator would take care of it.
* Changed to use mNumAppsPerRowAllApps that is already set in ctor.

bug: 316116551
Test: manual/presubmit/photos:
before: https://screenshot.googleplex.com/4Uo5Vtu3a6MDuGa
after: https://screenshot.googleplex.com/878g8yv5LgPsHaL
Flag: None
Change-Id: Ic481628b824c75e5578f3129e3a48f575c6a3655