Use device id 0 for touch/mouse events

Virtual device is only for keyboard sources, not mouse or touchscreen
sources. Here is the value of InputDevice.getDevice(-1).toString():

    Input Device -1: Virtual
      Descriptor: ...
      Generation: 2
      Location: built-in
      Keyboard Type: alphabetic
      Has Vibrator: false
      Has mic: false
      Sources: 0x301 ( keyboard dpad )

InputDevice.getDeviceId() documentation says:

> An id of zero indicates that the event didn't come from a physical
> device and maps to the default keymap.

<https://developer.android.com/reference/android/view/InputEvent#getDeviceId()>

However, injecting events with a device id of 0 causes event.getDevice()
to be null on the client-side.

Commit 26529d3 used -1 as a workaround
to avoid a NPE on a specific Android TV device. But this is a bug in the
device system, which wrongly assumes that input device may not be null.

A similar issue was present in Flutter, but it is now fixed:
 - <flutter/flutter#30665>
 - <flutter/engine#7986>

On the other hand, using an id of -1 for touchscreen events (which is
invalid) causes issues for some apps:
<#2125 (comment)>

Therefore, use a device id of 0.

An alternative could be to find an existing device matching the source,
like "adb shell input" does. See getInputDeviceId():
<https://android.googlesource.com/platform/frameworks/base.git/+/master/cmds/input/src/com/android/commands/input/Input.java>

But it seems better to indicate that the event didn't come from a
physical device, and it would not solve #962 anyway, because an Android
TV has no touchscreen.

Refs #962 <#962>
Fixes #2125 <#2125>