Improve TaskListener logging for clarity.

The log output for listener migration in
ShellTaskOrganizer#updateTaskListenerIfNeeded() was difficult to
interpret. Specifically, the toString() implementation of
StageTaskListener did not include the class name, making it hard to
identify the listener type from the logs alone.

This change improves the toString() implementations for
StageTaskListener and FullscreenTaskListener to enhance log readability.
StageTaskListener now includes its class name and formats its state as
key-value pairs. FullscreenTaskListener is simplified to return its
class name.

For example, the log output is improved from: "Migrating from listener
mId: SIDE ... to FullscreenTaskListener:TASK_LISTENER_TYPE_FULLSCREEN"
to the more descriptive: "Migrating from listener
StageTaskListener(mId=MAIN, ...) to FullscreenTaskListener". This makes
debugging listener migrations significantly clearer.

Bug: 387193964
Flag: EXEMPT DEBUG
Test: adb shell cmd protolog_configuration logcat enable WM_SHELL_TRANSITIONS
Test: Put app A and B into split then launch app A into fullscreen
Change-Id: I164f7b395a45f36474f3c2f099c31b0e73b94428