Use CastSysInfoDummy for fuchsia

This is a temporary solution until we implement a Fuchsia version of
CastSysInfo. The only downside to using the dummy is that the strings,
such as serial number, model, build, etc are all faked out.

Bug: internal b/126768956
Test: CQ
Change-Id: I3364d87c57d7f40dbe2d1e80b19bb2f02532512b
Reviewed-on: https://chromium-review.googlesource.com/c/1496419
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Auto-Submit: Aidan Wolter <awolter@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636644}
1 file changed