Input: evdev - flush pending events on clock type change

When client changes the type of clock used for the time stamps in input
events flush pending events from the client's queue (since client would not
know which events have old time stamps and which ones have new ones) and
and queue SYN_DROPPED event.

Signed-off-by: Anshul Garg <anshul.g@samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>