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- Add a install_immediately flag to ExternalProviderImpl;
- Set install_immediately flag to true for kiosk app provider
  so that updated app install is not delayed;
- Set auto acknowledge since kiosk should not show external
  install alert;

BUG=461419

Review URL: https://codereview.chromium.org/974713002

Cr-Commit-Position: refs/heads/master@{#319005}