Skip cancelling jobs with FLAG_WILL_BE_FOREGROUND.

When FLAG_WILL_BE_FOREGROUND is set on a job (such as a download), we
need to treat the job as if it had a foreground service running so
it can continue making forward progress.  We already ignore the device
idle state when offering to start the job, so this just avoids the
hiccup of stopping the job only to restart it a minute later.

Bug: 26571724
Change-Id: I348903dd3a7dd7104b0c1bf4310e2a48655d2588