timer: user_instance: use two signals to dispatch events explicitly

timer: user_instance: use two signals to dispatch events explicitly

Current implementation uses one signal to dispatch events. The user
application should distinguish actual type of event when handling the
evet. Nevertheless the type of event is decided by call of choose API,
before starting timer instance, therefore the application knows the
type of event in advance.

This commit adds two signals to dispatch the events to obsolete the
existent one signal.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>