Rewrite input transport using sockets.

Since we will not longer be modifying events in place, we don't need
to use an ashmem region for input.  Simplified the code to instead
use a socket of type SOCK_SEQPACKET.

This is part of a series of changes to improve input system pipelining.

Bug: 5963420

Change-Id: I05909075ed8b61b93900913e44c6db84857340d8