Fix a11y key event handling from multiple devices.

For M, we ignored device type entirely, but with ag/760625 we started
paying attention to it, which rejects events when keys from different
devices are pressed simultaneously. Since the volume keys show up as
different devices, the new behavior disrupted the event stream when
both volume keys were pressed.

Now tracking each device separately, which restores the old behavior
but still takes the device id into account. Holding down two keys
when enabling an accessibility service, however, always has and still
can produce an invalid event stream. It doesn't seem worth the
overhead, however, to track each key separately.

Bug: 28091773
Change-Id: I8d30de1f5e05f779b6fe305856d42f209ff8b038