Camera: Fix null pointer access

If a device is rebooted then the RecentTasks contains task list but
topActivityInfo could be null. In this case, if a camera device is
opened from a process without activity then CameraServiceProxy could
access null topActivityInfo.

Bug: 272428586
Test: Manual test with the OEM attention service
Change-Id: I9a9f42e8954d72e74c74f26ad5a690bcdd3a4cbc