Fixed sanity cache clear in User::saveSettings()

Fixed sanity cache clear in User::saveSettings()

* This works by adding a refresh mode to clearSharedCache()
  when we want to purge the cache in case it might stale to
  avoid further CAS errors. Because an exception will be
  thrown, the usual DB callback will not get fired, so avoid
  using commit hooks when doing these cache purges.
* Also lowered the tombstone TTL for such purges, since no
  data actually changed.

Bug: T114023
Change-Id: Iaad87b4ed24733dac40bc9607d3c97c940710087