[Sync] Rename HasSyncSetupCompleted to IsFirstSetupComplete.

This name better reflects what the value means. This change also renames
SetSyncSetupCompleted to SetFirstSetupComplete.

This change is working toward better control over when and how sync
starts up in order to fix the associated bug.

BUG=462796
TBR=sdefresne,sky

Review URL: https://codereview.chromium.org/1578443002

Cr-Commit-Position: refs/heads/master@{#370157}
61 files changed