[Sync] Rename syncable_prefs to sync_preferences.

Pretty mechanical CL:
- Moved files and ran mass-rename.py
- String-replaced "syncable_prefs" with "sync_preferences"
- String-replaced "SYNCABLE_PREFS_USE_POLICY" with "SYNC_PREFERENCES_USE_POLICY"
- Fixed a couple lints inside sync_preferences
- Ran formatter

BUG=659265
TBR=jam

Review-Url: https://codereview.chromium.org/2459823002
Cr-Commit-Position: refs/heads/master@{#428391}
197 files changed