UEventObserver: Fix logging for uevent messages

Every uevent message in buffer contains '\0' characters
and only part of the message is displayed right now.
So, all '\0' characters should be replaced for correct
logging.

Test: checked uvent message in logcat

Change-Id: I5bcdf117208feae542f0d4619cb03eddbcfce32c
Signed-off-by: Anton Utkin <avutkin@sberdevices.ru>