Add timestamps to FMQ ChannelMessage

Figuring out relative message ordering is harder with FMQ because
messages on different channels without timestamps do not have a
guaranteed ordering. This CL adds timestamps to all channel messages
to ensure relative ordering is always known precisely, and to make
timing more accurate.

Bug: 321810554
Test: atest VtsHalPowerTargetTest
Change-Id: Iac341dec2526ac46ae9db57aadbd267224a77989