Properly chain commands in MediaSessionStub

The commands currently use a task and a postTask that are chained
together manually. In some cases, e.g. when adding MediaItems,
the postTask is already a chain of commands in itself.

To allow using the entire command handling as a single task
(for simplified queueing), we can change the implementation to
always create a single task. If multiple subtasks need to be
chained together, we can do that by wrapping the method calls.
In case a task is asynchronous, we can also use Futures to
chain them together.

Overall, this is just a refactoring and changes no logic.

Issue: #85
PiperOrigin-RevId: 462085724