Add InputConnectionCommandHeader

This is a preparation CL to make InputConnection tasks cancellable.

In order to introduce a concept of sub session within a single
IInputContext, every IPC method defined there soon requires a common
data header.  This CL adds a placeholder object

  InputConnectionCommandHeader

for that purpose.

Note that this CL does nothing except for introducing such a
placeholder object.  Hence there should no observable behavior change
yet.

Bug: 195115071
Test: presubmit
Change-Id: Ib1465f4d80a6cc9a38e266995a163e5f0bef1115