New API: DevicePolicyManager.listForegroundAffiliatedUsers()

Bug: 173541467
Bug: 179163496

Test: m update-api

Test: atest CtsDevicePolicyManagerTestCases:com.android.cts.devicepolicy.DeviceOwnerTest#testListForegroundAffiliatedUsers_onlyForegroundUser,testListForegroundAffiliatedUsers_extraUser,testListForegroundAffiliatedUsers_notDeviceOwner,testListForegroundAffiliatedUsers_notAffiliated,testListForegroundAffiliatedUsers_affiliated

Change-Id: I520dc35da627d7cb89f6ff2d7010fc04af1b01f7