Use input transport for communications between app and IME.

The input method manager service now supplies an input channel for
communication while creating an IME session on behalf of the
application.

This change significanly reduces the overhead of IME event dispatch
by using a standard input channel to send input events rather than
using binder.  This results in fewer thread context switches
and fewer object allocations.

What's more, the IME may perform additional batching of the motion
events that it receives which may help it catch up if it is
getting behind while processing them.

Bug: 7984576
Bug: 8473020
Change-Id: Ibe26311edd0060cdcae80194f1753482e635786f