shell: remove /dev/input write access

Shell access to existing input devices is an abuse vector.
The shell user can inject events that look like they originate
from the touchscreen etc.

Everyone should have already moved to UiAutomation#injectInputEvent
if they are running instrumentation tests (i.e. CTS), Monkey for
their stress tests, and the input command (adb shell input ...) for
injecting swipes and things.

Remove the write ability for shell users, and add a neverallow assertion
(which is also a CTS test) to prevent regressions.

Bug: 30861057
Test: auditallow statement added in
  f617a404 hasn't triggered.
Test: ran getevent, saw correct output, played with device

Change-Id: Ia78eeec05f6015478dd32bd59505b51fef200a99