Only allow system to write to existing input nodes.

This prevents the shell user from injecting input as if it were coming
from an existing input device, which in turn makes it easier for malware
/ abuse analysis to detect when someone is injecting input via the
command line vs a real user using a device.

Bug: 30861057
Test: ran getevent, saw correct output, played with device
Change-Id: Ib53eea1b7767f25510b5d36fe6109101a9fad8e0