IMS-VT: Bring UI to foreground once video call is accepted

- If the user pushes the UI to the background immediately
after making a video call, it stays in the background even
after the request has been accepted
- After the remote device accepts the incoming video call request,
bring the UI to the foreground

Bug: 28767075
Change-Id: Iba2ebdd3c7f7e77fc8fe7ffc68897cbb6dd8610d