[PS] Show "Uninstall" in Long Press menu for Private Space apps.

Video : https://b.corp.google.com/issues/319250810#comment18

- Add SecondaryDropTarget.performUninstall()
- Make SecondaryDropTarget.getUninstallTarget() public static.

Test: Manual
Flag: aconfig com.android.launcher3.enable_private_space TEAMFOOD
Bug: 319250810

Change-Id: If6488033a976914fdc0a50658fc0561dc1a6586a