Return InputChannel from grantInputChannel

This will simplify the calling interface of this function, and many
other functions. But, more importantly, this will be more in line with
the 'move' semantics that we are trying to introduce for InputChannel.

Bug: 323450804
Test: none
Flag: EXEMPT refactor
Change-Id: Ib954f8fbf548555c2e0d4596924919b8ec9e699c