add clearFirst for load() to clear lookups.

[w-rui]

The load sequence of l10n delegates may not same with the sequence in MaterialApp(localizationsDelegates: [...]).
eg:
delegateA support en, ko, zh.
delegateB support en, ko.

MaterialApp(localizationsDelegates: [
  multiDelegate,

  delegateB,
  delegateA,
  ...,
]);

The code above works ok when locale is en/ko. BUT, when locale changes from zh => ko, delegateA overrides delegateB.
delegateA has been added to _lookups when locale is zh, and will not go behind of delegateB when locale changed to ko and add delegateB.

'clearFirst' will clear _lookups when locale changes and fix the problem.