Acquire WakeLock while processing Gateway messages.

This CL updates VcnGatewayConnection to acquire a WakeLock when sending
a message to the Handler queue. After every message is processed, the
state will check for additional messages and release the WakeLock if the
queue is empty.

Bug: 178140973
Test: atest FrameworksVcnTests
Change-Id: Ib108c834833cc8feb2c92c9a6c60f218618225ec