Use Result<InputPublisher::Finished> instead of callback

When the 'Finished' message is received inside InputDispatcher, we are
currently providing a callback function that gets executed with the
parameters that are matching the InputMessage fields.

This does not scale well for the case where there is more than 1 type of
InputMessage received from the InputConsumer. We would have to provide 2
callbacks, which is not user-friendly.

The calling code inside InputDispatcher is already aware of the
InputMessage struct, but InputMessage is intended to be a protocol of
communication between input channels, and is not meant to be used
elsewhere. To provide the output of 'finished' signal, we introduce a
new 'Finished' struct into InputPublisher. InputPublisher will now try
to read a message, and will provide a response in that struct.

This approach will also now force the caller to check ok(), which will
increase correctness.

Bug: 167947340
Test: atest inputflinger_tests
Change-Id: I301c6e9c39c02692f5565aa8affdcfd0e17bcefc