Introduce API to enable existing packages on shared users

- Also unhide setKeepUninstalledPackages
- installExistingPackage accpets delegation API because all app
  managemnet PIs did the same, including setKeepUninstalledPackages and
  enableSytemApp

Bug: 70017947
Bug: 65842106
Test: Install apps already installed in u0 in shared user should succeed
Test: Install apps in setKeepUninstalledPackages cache in shared user
      should succeed
Test: Install apps via delegated package should succeed
Test: Install apps via unaffiliated profile owner should fail
Test: Install apps not installed in any user or in APK cache shoudl fail
Change-Id: Iba563b2050abd0d1f46bfa06cfc0526b7b476b3b