PowerMode: Initialize arbiter in charging mode

The PowerMonitor already starts out in charging state at process
startup. However, currently the monitor is initialized after the
arbiter, which means the arbiter flip-flops between idle/charging modes
at startup.

This changes the arbiter's charging voter to start out in charging state
as well, which means the arbiter can't be idle until the PowerMonitor
is initialized, thereby preventing power scheduling policy application
during early process startup.

Bug: 1166695
Change-Id: I80378918b19b4751131c4e6151e6c39999796b1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2874609
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#879360}