Added removeUserWhenPossible API as SystemAPI

Also converted following APIs to SystemAPI:
android.os.UserManager.REMOVE_RESULT_ALREADY_BEING_REMOVED
android.os.UserManager.REMOVE_RESULT_ERROR
android.os.UserManager.REMOVE_RESULT_DEFERRED
android.os.UserManager.REMOVE_RESULT_REMOVED

Remaining task: remove all occurance of UserManager.removeUserOrSetEphemeral

Bug: 197181121
Bug: 199446770
Bug: 199446810
CTS-Coverage-Bug: 199446770
Test: m system-api-stubs-docs-non-updatable-update-current-api
      m module-lib-api-stubs-docs-non-updatable-update-current-api
Change-Id: Iea7867268950050f90d8509d7f3d09725d850a5e