PowerManagerClient: change suspend readiness API

The old API returned a callback which encoded the identifier for the
client's block-suspend request. The new API takes and uses an
UnguessableToken as the identifier for the request. There should be
no functional change. The new API will be easier to implement in
the Mojo client.

TBR=shik@chromium.org

Bug: 644348
Change-Id: I395f7746ad7075d1a994d4a08ebaaf267da1782a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575777
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654464}
18 files changed