Adding settings for managing auto-enter picture-in-picture.

- Add per-app setting for users to control whether an app can
  enter picture-in-picture when it is hidden.

Bug: 34520451
Test: make -j40 RunSettingsRoboTests
Change-Id: I182d6069ad01e42f1d3d6623ea21362ce802efba
Signed-off-by: Winson Chung <winsonc@google.com>