Use TaskContainer as source of truth around Task handling

- Renamed TaskIdAttributeContainer into TaskContainer so it's shorter
- Make mTaskContainers the source of truth for Task handling, removing mTask, mSecondaryTask, mTaskIdContainer and use mTaskContainers to derive all related information
- Remove mTasks/mSnapshotViewMap in DesktopTaskView and replace with mTaskContainers
- Mark methods that assumes a single Task as deprecated e.g. getTask, getItemInfo, getThumbnail to discourage further usage of such methods; "First" is added to the method name to make it explicit only first Task is considered; getTaskContainers is encouraged to access such information

Bug: 249371338
Test: Manual
Flag: None
Change-Id: Ieb15e0e8432913c89dc46c1011f0148095c665f8