Use WakeupMesage for delayed events in VcnGatewayConnection.

This CL updates VcnGatewayConnection to use WakeupMessages instead of
sendDelayedMessage() for delayed events. This change allows for
VcnGatewayConnection to guarantee that all its messages are processed
while holding WakeLocks.

Bug: 178140973
Test: atest FrameworksVcnTests
Change-Id: If8182378fe2e9dad08df88a84934b21e22e25752