HdmiVideoExtconUEventObserver: handle "DP=1" state

HdmiVideoExtconUEventObserver handles only "HDMI=1" external
connection state while kernel generates uevent with "DP=1" state
when external hdmi display is connected via USB-C-to-HDMI converter.
This results in wrong DisplayPolicy.mHdmiPlugged value when
hdmi display is connected, so we need to handle "DP=1" state as well.

Bug: 288003290
Test: take bugreport while external hdmi display is connected
and grep for "mHdmiPlugged=true"
Signed-off-by: Aleksej Makarov <aleksej.makarov@sony.com>
Change-Id: I410122aad09a5414a736cac51647e75fcfa61055